Hey Bounty, I'm sending two extra letters, and I think I'm going to mail each school a letter beforehand letting them know that two extra letters are coming, and that I have waived any right to view them. Also, since they probably won't be done until all the schools have everything else, I think I'm going to request that they begin processing my application once the minimum set of documents is in, and then review the extras as they come in. I specifically asked my letter writers to wait until a certain day before mailing them out, so I would have time to get that letter in.
But I know some schools specifically say NO EXTRA LETTERS. For example, Tulane doesn't want anything extra, and Wash U says you can send only one extra, so everybody should definitely research the schools they're applying to.
Also, some schools, like U. Vermont, specifically say that they will not review your file until they recieve ALL letters, and asks you to list all the letters that will be coming.
Good luck!