Depends on where you stay. I've been pre-upgraded to suites multiple times. At Hilton Reforma Mexico, they pre-upgraded me to a master suite, which is the best room type in the hotel. When they then tried to downgrade me at check-in, I (successfully) insisted on keeping the suite I had selected. At ES Sacramento, they would consistently open up all rooms for me as a Diamond the morning of arrival, which would allow me to confirm one of the very few river view executive rooms that does not front on the elevators.

Most recently, I declined to use mobile check-in at Doubletree Modesto, and by the time I got there, virtually everything was gone and I wound up with one of the worst rooms in the hotel. Lesson learned.

Great responses! I decided to OLCI and there were plenty of rooms at the rate I chose (no upgrades). Compared to the pre-assigned one I was able to move up a few floors and change sides for a possibly nicer view. I might still ask at the counter when I arrive; mostly I want to get a room with 1 king bed since Concur forced me into a 2-bed for price purposes.

Good topic OP! I rarely use OLCI as I don’t know the properties well enough to distinguish the good side of the hotel from the bad one, and Hilton follows instructions in my profile to keep me away from elevators and ice machines. I’ve also feared that locking in on a generic room might prevent me from getting an upgrade upon arrival. Nothing here suggests that I should change my practices.

My wife and I do not like noisy upstairs neighbors, so I like the ability to grab a top floor room. Even though we have "upper floor, away from elevators" in the profile, there have been too many times that the pre-assigned room is just the opposite.
I will typically pull up Google Maps / Earth and check the potential view, plus the proximity to highways or other noise (even morning / evening sun), to help decide on a side.
Beyond that, at Hampton or HGI which are 3 or 4 floors, we will use stairs much more than elevator from our top floor room, so I like to be near the stairway -- but not right next to it because of the banging door. (and yes, I try to be respectful of those who *are* in that room)
But like others, I typically wait until the morning of check-in, because of a *few* occurrences of pre-upgrade. Recently I have also been seeing the available inventory become much larger in the morning compared to the day before.
That being said, if inventory looks scant and if being top-floor is important, I'll grab it the day before. That circumstance is more likely if I book something nicer than the basic room, and I know that the supply of that room type is limited.

I haven't historically used OLCI, because 1) how are you going to get your breakfast vouchers/perks without going to the front desk anyway, 2) the hotel doesn't have Digital Key so you need to go to front desk anyway.
Perks being the main thing - don't want to miss out on them just cause I checked in and went straight to the room....

You would have been severely disappointed with my check in experience last week:

1) Was told just to give my name/room number at breakfast, no coupons.

2) Was told that the wifi was down, but they were hoping to have it working soon. I do hope the guy who snapped at her about getting compensation got his 500 HH points.

3) Was issued 2 plastic room keys that didn't work. I had also downloaded the digital key, so at least I didn't have to schlep my bags back to the front desk.

The funny thing is that this was my first stay as a Diamond and the check in was quite underwhelming. I was at the property for less that 9 hours, so I really didn't care. Just wanted a quiet room with a comfortable bed and I got both.

After a whole year of OLCI I have decided no more. During 2018 I was only upgraded twice. And that is when I did not do online check-in. Go figure.

30 stays, 60 nights per year with Hilton for years, give or take, but always Diamond. Hard to screw up what I've reserved for a night or two, and I've yet to get screwed by the hotel. My demands are low, my reserved room and that is it. If I'm spending time in the room I'm not working, couldn't care less if its floor 12 or 20. Realized that this last weekend when I selected one room, got upgraded to another, but moved to the same room on the 12th floor. No big deal for me.

I got upgraded again yesterday. I finally did the OLCI when I was at the airport waiting for my flight.

Not as exciting as an upgrade in Europe or Asia but it works for me.

My conclusions after 77 Hilton nights this year are pretty similar to others:

1. OLCI if you get a good upgrade. Nothing's guaranteed, but it puts a moderate barrier in the way of the property subsequently moving you.

2. OLCI if you are worried about your arrival time and want to salvage an acceptable room at the cost of a low upgrade chance. However, a phone call goes a lot further if you want your room to be held.

3. Never accept the barefaced lie that FD can't change your room once you did OLCI. Definitely worth politely pushing back against such a claim.

4. Despite 3., don't OLCI to an indifferent room unless 2. applies, since it might take you out of the running for a pre-upgrade, and worthwhile upgrades might all be allocated by the time you arrive and push back as per 3.

5. Marriott OLCI is a bizarre parody of a check in. Since I'm on property at a Renaissance at the moment I really should ask a manager in a quiet moment what it means on their side of things, if anything!

My strategy- I watch OLCI like a hawk from 6am the day before , usually here in the uk they do upgrades the evening before or the morning of stay.
As soon as upgraded I then do OLCI to try to lock in a great room ( I do lots of research! ) .
If I'm not upgraded I wait and ask politely at front desk on arrival.
The only time I OLCI before an upgrade is if I'm arriving late or have a favourite room at the particular hotel......this is nearly never.

I still do OLCI if I know I'm checking into a large/busy Marriott. Yeah, theirs is not nearly as good as Hilton with Digital Key, but the hotels that are likely to have a check-in queue sometimes do have an express lane for OLCI. It has saved me a half-hour on a couple of occasions - Philly airport late at night, one of the big Chicago Magnificent Mile properties, and a convention hotel in San Diego. Marriott never upgrades me (even though I'm LTPP) so upgrade strategy doesn't even enter my mind. At a legacy-Starwood that might still have a bit of an upgrade culture, I'd probably hold off...

Just getting Digital Key is worth it to me. That way I can walk in, take a look at the front desk, and decide then if I want to talk to them about anything. If they're busy, I can go to the room and go back later to figure out breakfast coupons.

It might not be a "can't" change situation, but I do believe that if you do OLCI, they aren't going to touch your selection unless they have to. I've been specifically told by a front desk manager, on the phone the day prior to my check-in asking some room layout questions, to *not* OLCI because it throws me out of the upgrade pool. He said "let us process the Diamonds at 10AM morning of arrival, then you OLCI. We'll have you on the Exec Floor and you'll be able to pick from a larger selection of rooms."

I still OLCI most of my regular business trips the day prior. If there's high-floor, away from elevator, King bed...I take it and usually have my digital key by noon. A solo 2 or 3 night trip...I'd rather have assurance of a quiet room than a 2% chance of a large suite that I don't really need.

I think this is YMMV with specific properties. I don't have enough experience as Diamond to comment on Hilton specific properties. With Starriott I've twice had to call and cancel after OLCI over the years.

* Marriott In Airport Calgary - booked using Chase Anniversary night - last flight of the day to YYC was cancelled due to weather; called front desk; gal cancelled check in AND offered to get in touch with customer service to get my anniversary night award refunded.
* Renaissance Reno - prepaid 1 night - Hwy 80 to Reno closed due to weather - front desk gal cancelled check in and refunded back to credit card without any hassles.

I do have to add that I think the OLCI ability to pick a specific room on Hilton app is head and shoulders over OCLI offered on Starriott app.