It is best to make a breakfast PS on a non-EE day, especially in MK. If you do it on an EE day, you basically miss the EE hour and it defeats the purpose of going to EE. Then when you get to the rides, the park is extra crowded.
When you do an 8-8:15 ps, you get to go on the rides as soon as the park opens and you get to walk down main street when it's practically deserted, which is pretty cool.
If you have to make a PS on an EE day, I would make it for the latest time possible--10:30-10:45. That's about the time the crowds start getting bad anyway, and we usually hop to another park around noon.