I was under the impression that they allocate rooms ahead of time, so requests will have been considered, if possible, when they assign rooms a day or two before. I am begining to wonder now, though, if perhaps they just give you the first room available in your price catergory. If you aren't happy with the room they have given you at check-in, in my experience, they will see if they can switch, but you might have to wait a little longer for the room.
At the WL I had been really good at making sure that my couple or requests where in the system ahead of time, but when we got there they had put us in a room with bunk-beds, which are usually hard to get for other families. My DS4 is just too much of a live-wire for bunkbeds to be safe, so I asked for us to be moved. We waited about an hour, and we were moved to a room that had most of my original requests honored, so I felt that I hadn't done too badly. I try to keep my requests simple, no more than two if possible, I want to be flexible so I wont be disappointed.