AFAIK the old PMNW trick of not checking in while waiting for upgrade inventory to open doesn't work with DL.

However, it is also NOT the case that an earlier check in time moves you up on the wait list for gate upgrades.

IMO if you can conveniently check in just before going to the airport, I would do this. You can watch for the upgrade (or for upgrade inventory to open) and if you're upgraded, you might be able to change your seat on line to one that you prefer. I don't see any advantage to checking in early, around T-24--except that you can then see where you are on the upgrade list and agonize as more elites check in--provided you don't risk missing the deadline to check in for your flight.

Note that having already checked in does not protect you from being automatically rebooked against your will to flights you do not want in IROPs.