I also read, either on another board or in a newsletter, that certain resorts are no longer accepting faxed requests. I think it mentioned WL, but I'm not entirely sure. The article said that requests must be noted on the original reservation in order to be considered.
With regards to faxing requests, we've had luck sometimes, but not everytime so we really don't bother anymore, we just ask at check-in. At ASMo we requested Toy Story in a fax and got a great room there, overlooking Andy's Room. The 1st time at AKL we made our requests over the phone since the fax # wasn't working so the CM who answered said he'd write down our requests and gave us exactly what we wanted, 2 rooms on the Zebra Trail, 2nd floor, one on the Arusha and one on the Uzima Savannah. The 2nd trip, however, we got a horrible room on the Sunset Savannah, overlooking a people pen on the 5th floor, plus about 6 trees directly in our view of the giraffe feeding station. We hardly ever saw animals, only around midnight. ASMu, we didn't fax anything, just asked at check-in for a pool view (we had a preferred room) and we got a great corner room on the 3rd floor, facing the pool and right next to the main building.
So really, I think it depends on a lot of things, what you request, how busy the resort is, and how specific your request is. I think there is just as good a chance at getting your requests at check-in as if you fax a request.