On the TP website, they recommended (via a question on the forums) NOT to put in any other requests -- by phone or during online check-in -- if you're going to have them fax. As mentioned, they'll automatically send the fax 5 days before check-in. You'll get an email telling you it was sent. If it doesn't go through for some reason, they'll continue trying to send for 24 hours.
Also, we booked both our hotels through Orbitz, and, prior to finding out about TP's fax service, I called Disney and tried to add requests. They wouldn't let me because I needed to go through the "
travel agency" I booked with. TP's fax got through with no problem, and I even got a reply to the fax from POR letting me know they had received it. So, it seems like a great option for third-party bookings.
Apparently, the wording TP uses is quite different that what we "regular" people would use. They claim that they researched Disney's own lingo to use the specific wording that the room assigners use, to increase the possibilities of getting the room you want. The requests are something like:
Bldg 1,Flr 5,NW-facing room.Exmple:5155,5157,5153,5151,5159,5149,5161,5163,5147
You can go to their website and look at the room views there for free, and at the bottom, it will give you the wording to fax if you want to do it yourself.
I was skeptical -- until it worked so well for me at two resorts in March.