I've always been told the buses start up a 6:30am since it's the only way for them to accomodate guests with early reservations for meals and tours. Chef Mickey's and a few other spots start seating as early as 7am (many others start up at 7:30) so they kind of need buses early for things like this. We've never been out quite that early, but we have been out between 6:45-6:55am and caught buses no problem. You can usually just tell the driver of any bus that you're trying to make a character meal and they'll take you wherever you need to go...of course, for CRT an MK bus works too. Case in point...we had an 8:05a at CRT back in Nov...were running a tad late for my tastes that morning, but got an MK bus right around 7:30a and were at MK in plenty of time to make it to the meal.