As someone already stated, they try to meet requests based on when you made them - it makes sense to make your requests when you make your reservation. It's also been reported that if you call & modify your reservation (to change something or simply to add a request), then your *date* changes to that date, and that's the date used for determining when your current requests were made. For that reason, I make my requests when I book - at 11 months out. I've never been disappointed, in years of 6 or more trips per year.
I personally see no reason to waste my time or Disney's faxing requests prior to arrival....even at non-
dvc resorts I make my requests at reservation time and almost always get what's been requested.
I have called the morning of my arrival to see if my room is ready (meaning it would have had to have been empty the night before) if I'm already staying on property somewhere, and asked about changing to a different room if mine's not ready but others are. So, while a cash customer will be at the bottom of the request list (along with last-minute dvc bookers), it costs nothing to make a request, and due to last minute changes it just might happen that the stuff requested can be met.
I'm not sure if they give priority to dvc members if they've got 2 reservations both made at the same time....I would think so, but I'm not sure. There are so many wonderful rooms at the dvc resorts, I can't see obsessing over it that extent!