It really depends on the game and its rep on just how fast it'll fill up. Don't quote me on this but I think it was last year (or was it the year before) that Can of Whupass - a 20-player game - filled up fresh out of the starting gate.
Pre-registration is the way to go if you want to guarantee your slot in a game. If it doesn't matter, if you want to throw the dice to see if you can get in to a game, go with the generics. And if you change your mind about a game you've pre-regged for, you can always stop by the Customer Service desk and return it. (Just do so well before the game starts and please don't forget about the return fees (5%, I think).)
Please also note though that the construction and subsequent scattering of the events last year threw things off a bit. A game I was in was sold out through pre-registration but only half of the players showed up, offering lots of opportunities for those with generics.
We are here on Earth to fart around. Don't let anybody tell you any different.
- Kurt Vonnegut, A Man Without a Country