I think it depends on the child and what your intentions are. If you are willing to leave the parks or skip rides because DD is hungry, tired, needs to go potty, overwhelmed, etc., you'll be fine. My DS turned 3 right before we went in April. He had a blast and still asks when we're going back. We gave them the choice throughout the day to ride or leave. Luckily he never wanted to leave, but if he had we would have been out of there.
We did lots of planning. In MK we did the Dumbo or Die touring plan from the Unofficial Guide. We rode everything we wanted to and were never stressed. Also, DS didn't like the fireworks, so we took off for Fantasyland and walked on all of the rides. Just be in tune with DD and have fun!