There is another thread on here somewhere (I tried to find it but couldn't) where this is being discussed. Some folks believe it happens a day or 2 before check-in whereas a lot of other people are of the view that it's at check-in.
I'd tend to support the "at check-in" view but if they do have your reservation and you want to politely ask for something (like a particular view etc) then I don't see the harm in ringing shortly before your stay and asking.
There's another thread where people are saying that Disney may not discuss it with you as if you went through a TA, the reservation won't be in your name until you actually arrive at the desk. I don't know the truth of that one but it can't hurt to ask...
If you booked directly through Disney you can add a request just call and add it... If you booked through a travel agent you may wnat to ask if they can.
Disney is supposed to use room ready now. This means it is assigned at check in. I would ask at ths point as some are flexable and some are not. In the past you could fax a request but this is no longer accepted.