Other than for Southwest, checking in early is a precaution that only Flyertalkers would worry about, due to remote chances of IDB, or for upgrade list priority. If none of this applies to you, then don't bother.

The only time I ever check-in early is if I'm running late and want to bypass the kiosk. Otherwise, checking in late gives you the max flexibility to change flights/seats online, as some airlines lock-out options after check-in.

checking in early (and printing your BP):

1. can alert you to problems in advance, such as a magically disappearing seat (even though you reserved months in advance and picked a particular seat, it can disappear for no apparent reason). knowing this before arriving at the airport allows you to plan or call the airline to try to work something out.

2. theoretically makes it more difficult for the airline to pretend your reservation doesn't exist.

3. gets you past the check-in counter, through security, and to the gate, where you have much more leverage against GAs who are pressured to get the flight out on time v. check-in agents who are under no time pressure.

4. avoids the risk of the 30 or 45 minute cutoff for checking in.

5. affects boarding order on southwest, possibly affects upgrade priority on certain airlines (but only in very rare tiebreaker situations), and apparently affects the priority for IDB as others have stated.

"they must first have asked for volunteers for VDBs and gotten no volunteers. "
i do not think that to be correct.

when a return flight from europe to usa is cancelled, IDB's on the next flight are all over the place, and the airline does not treat the "guests" politely. i try not to fly out of fra or muc in winter, because of so many cancels, and the idb's get abused.

This.

Now and again I have problems with a connecting flight - No boarding pass will print, itinerary problems etc. 24 hours gives me time to fix the situation.

I checked in early on our last flight and realized that they had scattered the "kids" - 16 & 17 all over the plane. Called and got it sorted but wouldn't have been able to if I had waited. Did they HAVE to sit together, no but they wanted to.

Back in the days before online check-in, I was scheduled to fly LGA>MSP and arrived at LGA quite early, due to the limousine schedules from where I was living. Just before the scheduled boarding time, we were told that the flight was cancelled due to mechanical problems.

However, they continued, both the first-class passengers and the first twelve coach passengers who had checked in could be accommodated by taking a flight to ORD that was leaving in 30 minutes and transferring to a flight to MSP there. The rest would have to be shoe-horned (they didn't say "shoe-horned" but that's what they meant) onto other flights throughout the day.

I was one of the twelve who got to leave on the ORD flight. :)

Another time I was supposed to fly out of MSP, but the flight was cancelled due to a blizzard. When the airport reopened and I was given a new flight number, I got there good and early, because I did NOT want to end up in a middle seat. I was so early that I knew I would have to sit around for a long time (hey, more time to read), but I was rewarded when they handed me a boarding pass labeled "2A," my one and only op-up so far. (I had a lot of NW miles at the time, but not an unusually large number.)

So if you check in early, the potential rewards are great.

I see your point. But in that case, the real problem is rebooking the pax on the cancelled flights to new flights. I haven't experienced this situation on int'l flights, but have on domestic flights. It's a mess. But if the next flight is full, pax from the cancelled flight are not going to be rebooked on that flight.

Related to ksandness's story: I was once flying STL-LGA, and my flight was cancelled due to the inbound plane stuck from weather. I was in the Admirals Club, got the notification on my phone, and got myself rebooked on the next flight. Then I took a walk around the terminal, saw massive lines at the gate and at the "AA Self Rebooking Center," and was extremely thankful I wasn't in that situation. Since the next and last flight was pretty full to begin with, I doubt many of those got on the flight. And I doubt anyone was IDBed to make room.

The above two reasons are the best reasons (IMO) that you should check in early. The latter reason being even more important if you don't have status on said airline.

Not sure where your data is coming from, but in 2011 48,000 passengers were IDB'd on the major US Airlines. Granted, there were 11 times as many who took the voluntary, but I would not call it unlikely by any means.

For me, the biggest reason is getting on the UA upgrade list.

This Friday/Saturday I toyed with changing flights to get home earlier on Saturday. Not being 100% sure, I didn't change Friday night but waited until about Noon on Saturday (about 4 hours before my new flight). When they cleared all the upgrades I wound up missing it by one seat. I'm pretty sure that I'd have made the cut if I'd changed and checked in Friday night rather than Saturday noon.

What? Where did you get that little nugget from?

Agreed, checked in late recently at kiosk and was offered to take later flight for 300.00 before even going into the airport. Sometimes it works both ways, depending upon what you consider a benefit.

1. can alert you to problems in advance, such as a magically disappearing seat (even though you reserved months in advance and picked a particular seat, it can disappear for no apparent reason). knowing this before arriving at the airport allows you to plan or call the airline to try to work something out.

This - it has happened more than once that I have gone online to check in, and my booked / paid for seat has changed itself into something much less desirable! Usually though I can self-rememdy by selecting another seat at that point.

I hate it when I think I'm gonna get an upgrade and in comes the people checking in 10 minutes before the upgrades happen