You'll get a lot of varying advice on your questions. Some people swear by the faxes, others will tell you that they get thrown away because the room assigners are "too busy" to mess with them.
Here's my experience. In 2007, I took my wife on a surprise 10th anniversary trip to the WL. I had booked a standard view, king-sized bed room. A few days before we arrived, I faxed a letter to the WL stating how excited I was for the big trip and how we had honeymooned at the WL in 1997. When we got to our room we had been upgraded to courtyard view. We were right above Artist Point's outdoor eating area with a view of the pool and Bay Lake. We also found two towel swans forming a heart on our bed. Was this the result of the fax? I'm guessing yes.
In the summer of 2006, we took the kids and my mother-in-law to ASMovies. I did not send a fax there. My requests when I booked were Toy Story (booked before it was preferred, arrived after the policy change) and connecting rooms. The CM who checked us in didn't mention if our requests were met, so I asked. "Of course they were, you had them noted in your reservation." We also did not have to pay an upgrade to the preferred room rate because we booked before the rate change. We ended up in connecting rooms right outside where RC is parked in the Toy Story section.
YMMV.