Online check in puts an update in the central reservation system, that info should properly get transferred to the resort 5-6 days prior to your arrival so that the room assignment team can work to assign the rooms.
In other words, I would do it now so the info can properly be transferred from one system to the other to make it to the resort systems before the 5-6 day mark.
I say "should" as there have been issues lately with guests having options on their package reservation and when they are checking in at the front desk they are told the options aren't on their reservation. Always take screenshots or emails and printouts of your confirmations with you to the front desk.