Unless you're looking for some beds to flop in for some rest, or want to hit the pool, you may want to hold off on getting a room early. Generally speaking, the rooms that you'd be likely given early would be ones that weren't used the night before. Keep in mind that the CMs at the front desk normally like to give out rooms that are more desirable first, and hold onto the "dumpster view" rooms until last. Therefore the unused room you MAY get early might be one of the ones that they refrained from assigning before your arrival. However, as mentioned above it's still possible to get a good room early too.
Unless you really want to get into a physical room early, I'd do a "pre-check-in" instead. They can do the full check-in (minus the room number), give you your keys, tickets, maps, etc. and you call the desk after 3 PM to get the room number. In the meantime you can give your luggage to the bell desk and it will be delivered to your room while you go off to the parks, or go shopping, or go have lunch somewhere.