People that have worked as CMs in the resorts before have stated here that there is no single policy regarding room assignments at the various WDW resorts. However, from their experiences the only resorts that routinely pre-assign rooms are the value resorts, probably as an effort to process check-ins as quickly as possible. They have said that usually the other resorts won't do a pre-assignment unless it's due to a specific request related to something like a medical condition. But even if a room is pre-assigned, in most cases a front desk CM can "snatch" your room and give it to someone else as long as they re-assign you somewhere else. On top of that, the available room inventory is dynamic due to things like rooms being taken "out of service" due to maintainance issues, people expending their stays, etc. that can trigger a domino effect of changes to rooms that may have been pre-assigned. To me, all of this makes the whole pre-assignment issue pretty much "moot".
As a side note, things in this arena may have recently changed. I read on the WDW update at Mouse Planet that WDW just won an award for a new check-in system that speeds things up by automating much of the check-in process and allows the CM to interact more with the guests at the desk instead of spending most of their time with their faces down working on the computer. Perhaps they've gone to more of an "automated" room selection process.