It depends a lot on the con, I'd say. I would have had to cancel my CoC game at Springfield GAME is there hadn't been a bunch of RPPR folks available who wanted to fill the seats. But at Fear the Con 7 back in June, almost all the tables were full for just about everything.
If it's a small con or you're friends with the organizers, you might want to talk over how they could improve their system for signing up for games. Fear the Con had an awesome online pre-registration system, but Springfield GAME only had an online list of events, and just on-site paper sign-ups. I think that made part of the difference. (Well, that and Fear the Con is a 100% dedicated tabletop con.)