The only WDW resorts that still read faxed room requests are POR and POFQ. So if you are not staying at either of those, your fax won't even get read. If you are staying at one of them, you can send a fax. It is helpful to the room assigner to know why you are requesting a certain area. The fax gives you more space to describe what you want. If you do send a fax, send it about 9-10 days out. The room assigners get the reservations 5-6 days out.
If you're not staying at POR or POFQ then having your requests added to your reservation is all you need to do.