"best" is what works best for your family. Many people with little ones like early because there are good activities in the club/lab during late seating, and many little people don't stay up late. However, if you are coming from the west coast, late dinner may keep your kids more on their "at home" schedule.
The earlier you book, the better chance of getting your requested time. Of course, summer 2007 dates/rates aren't released yet....keep watching the internet....