By all means check in early, I've checked in as early as 9am and been given room keys, moved in and bobs your uncle, hit the parks by 11am. Member since 2000, checked in at 9am, 10:30am, 11am, 2pm and to date I have never been told to come back later at any resort and I've stayed at them all in Orlando except BLT.
I've read the horror stories about check in but have yet to experience it, I guess I should knock on wood!