My preference for before parties is to enjoy counter service instead of a sit down meal.
The one year we did the Christmas party and ate an early dinner at the castle (4:50pm). Although the food was great (it was pre - pre-plated) we were so full that we had difficultly doing everything I wanted to do at the beginning of the party.
Lesson learned, all three times I've done the halloween party, I've stuck to counter service before the party. I enter the park at four, hit the rides that are not open during the party, once the party starts do as many of the special things as possible, then grab some food / snacks and watch the parade.
when at Disney I try to book my sit down meals where I don't plan on doing much afterwards (like relax at the hotel or find a spot to watch fireworks, or meander around and do some shopping).