We did this last October - we had an 8:05 reservation at Crystal Palace. The buses started running very early and made stops at MK, Poly, GF and Contemporary (first stop from Pop was Contemporary, then MK). I forget what time we got on the bus, but we were so early that we stood at the gate for at least 20 minutes before they let everyone in. They let everyone in at exactly 8:05. I have read on other threads that they check a list to make sure you are supposed to be in the park that early, but that morning they did not (perhaps they were running late and that is why they didn't open the gates until 8:05).
Don't worry about it, I worried so much after I made that reservation and it was for nothing!! When I checked in I talked to a CM to make sure of the time the buses to the breakfasts started and everything went so smoothly!