Your trick of looking at the C+ load T-121 is solid. While I’ve had upgrades clear at T-72 as PM, my overall experience has been that I either clear at T-120 or at the gate with little in-between.

As others have mentioned, Domestic DeltaOne routes (i.e., HNL) are a clear winner—especially when confirmed at booking (I cleared four for me + companions recently for a red-eye).
If you have trips with multiple connections that can clear at booking, that is another winner.
Another good use is when traveling with friends who are on a separate reservation when your Medallion upgrade clears and theirs doesn’t and you use the certificate to pull them to the front.
I never burn a RUC on a flight that is so short I don’t get a meal. Anything to the West Coast or LAS is a good call.
It’s sometimes fun to use a RUC when there are 75 on the upgrade list and you want to make the DM who is #1 on the UG list gasp when you clinch the one seat available. I did this last year only to find that the person below me on the list then used a GUC to get back to #1 (and then two seats became available, which meant the burnt the certificate for nothing).

I used to fret about not wanting to “burn” the certificate on an upgrade that might otherwise clear on its own at the medallion window. I’ve gotten over this and had no hesitation using certificates when I can clear at booking as it’s nice to have a bird in the hand so to speak.