The ONLY reason I would wait to book is because of the small, small, small possibility that they don't allow you through the screening process after registering.  If you are a repeat NESTer, I don't think that's an issue at all.  If you're a first timer, well, there might be a 1% chance you won't be allowed to go.  Tip: That might happen if you admit you're a murderer or something like that. 😉