if you think they're worth more than 18% sure, leave some more. If you don't, then don't.
Keep in mind that at some of the non-disney places, they get 18% of what Disney pays the place rather than 18% of your bill. If you think that's enough, then don't leave any more. If you'd like to leave more, leave more.
I personally bumped one server up. The rest I didn't. And I wouldn't even think of bumping someone up at a buffet. 18% is more than sufficient for semi-service.