A lot of people actively work on their application until the submission deadline. If your product is moving fast, along with adoption, it would be handicapping yourself to submit weeks early, I think.

Actually several of the best startups we've funded applied at the last moment.

A lot of people didn't know about this until recently though, like me. I'm not in any danger from Sandy right now, though and still want them to be able to submit. Obviously a deadline extension would be positive for me, but I was prepared to submit tomorrow regardless of seeing this post.

Edit: Maybe you filter by location? But why make it complex?

Re: rdl's comment, I'm not sure. I'm not experienced with YCombinator process or startups, but can't the founders explain their application was sent as of the submission date (weeks or days before), and then explain during the interview about the important changes? I mean, Parkinson's Law is in play with entrepreneurship big time.

I know lots of YCers who worked on their applications until the last minute, myself included.

Some people (like me) might be working on demos to go with the application. I also dwell on details like appearance and usability, and am hesitant to submit something I am not really happy with. I am going to submit right at the end.