One the whole it's probably better to update the application incrementally. You'll get more attention that way; we may even suggest ideas...
This statement sounds like the YC team reads unsubmitted applications. Have I misread? Anyone have hints on how to maximize the chance of getting comments back? :)
I was assuming by "apply" jsjenkins meant submit the application, since you can resubmit as often as you want. Since we read applications online, we see the most recently submitted version of any application we look at.
One the whole it's probably better to update the application incrementally. You'll get more attention that way; we may even suggest ideas; and we're not likely to have time to do that if you apply the day before the deadline.