They will fill from Scar and then to the left (Sebastian, Ursula and Witch are reserved for the dining package). The best section is Mickey as it is in the middle but you do get a good view from pretty much any character, just make sure you dont sit in front of a post!
When I was a CM we didnt used to make everyone move to the end of the bench as we didnt want to annoy guests but now they make you move to the end (obviously because they have to try and fit as many people as they can in now with the shows being cut) .. anyways, if you wonder around and keep your eyes peeled, you will usually find large gaps between people that you can sit in!
I would say be there about 90 minutes before and if there is a second show, go to that one (But the atmosphere from the crowd in the first show is awesome!)
Hope you enjoy it!