Here are my experiences, with requests and how they were made. I admit, I've been lucky, though.
POR...booked standard. Requested Building 14, via fax. Got it. September.
Poly...booked Garden View. Requested Samoa facing Volcano Pool when I reserved. Also faxed a few days before. Check in..upgraded without asking. September.
POFQ...booked standard. Can't remember what I requested but I did it on the phone when I booked. Tried for a couple of weeks to upgrade to water. When I checked in, I inquired about upgrading (paying) and they just gave it to me. I don't know if my original request was granted, I didn't ask, I really wanted to get the water view I had been trying for. January.
Pop...booked standard, through Travelocity, so I couldn't note my request, I had to wait until 5 days before arrival. Day 4 I faxed. Requested 70's, when I checked in I asked if my request had been received, CM verified that it had and I got what I asked for. May.
WL...booked courtyard for only one night, through Travelocity. Called the hotel directly 4 days before arrival to request direct lake view, upper floor. When I checked in, I asked if my requests had been received. CM said yes, and not only were they fulfilled, you are in concierge. May.
AKL...booked Savannah, faxed request for Zebra Trail, 3 or 4 floor. I inquired on check in if request was received. She said yes, but "not guaranteed" and I said of course, I know you do your best! She seemed appreciative and told me she could give me what I asked for, but construction would bother me, I told her I trusted her and she found me a great spot, ready right away at 10 AM! May.
I know I've had remarkable luck. Part of it I believe is the times of year that I go. Part of it is being polite and letting them know off the bat that I don't expect anything, that I know they do their best and I'm just happy to be at Disney! Did I enjoy my granted requests? Of course. But had they not been fulfilled, it would have been fine with me, I wouldn't have pressed it unless I ended up with a truly wretched room.
In my experience, faxes probably aren't necessary unless it is in the case of booking through Travelocity or another service. If you book through Disney, just put it on your reservation and ask if your requests were received when you check in. Whether I requested on my reservation by phone, or by fax, Disney got it.