It all depends.

AA uses check in time as consideration for SWU upgrades, but not domestic sticker upgrades. So if my domestic upgrade hasn't cleared in advance, I won't check in, because if I check in at T-24 and my upgrade clears after that, the system won't let me change my seat anymore. It seems that AA has now added this functionality with the latest update in its app, so it shouldn't be an issue anymore.

Checking in early may reduce the chances of being bumped off your seat or your flight involuntarily, but that's really not something that happens often and it's not something that I concern myself with at all.

If I get no benefit from check in time (if I am already confirmed in F, etc), then I will just check in whenever I feel like it.