I don't bother checking in online early, because I go super early to the airport and my flight leaves very early. If you are leaving mid-morning on, I would do so.
Also, don't check in on the way back if you plan to use RAC at the resort. If you check-in ahead of time, it bumps you out of the computer, because essentially you have already checked in and you have to take your bags to the airport. A girl in front of me did this last year, and boy was she livid when she got to the RAC desk. But it makes perfect sense.