The times when the dining room is really busy is right after the show. That's when they often give out the little "beepers". It takes some time to seat everyone as they pour out after the show. We find that if we want to eat late after the show -- we're better off to go for a drink or a stroll around the deck for a half hour, then we can usually walk right in.
In my experience, before the show isn't that much of a problem because people still arrive at staggered times.