DH and I will be there beginning 9/15, and we've only made a few PS's, those being for the more popular restaurants (Artist Point, Le Cellier, etc.). That way, we know we'll get in when we want and not have to wait. We don't like to be tied into eating at a certain time, or planning so far in advance not knowing exactly what we'll be doing or feel like eating that day. But remember that you can also call for a PS on the same day, using the park phones.
September is not usually crowded, but with the FTP who knows??