I have to ask..why would Disney Dining ask if you knew what room you were in yet? Just curious.
As for when rooms are assigned..to the best of my knowledge, they are blocked out about 3-5 days in advance. But, nothing is etched in stone at that point. If you're standing at the front desk, checking in, and you find that the room you have been given is not acceptable, many times you can get a different room. Until you get that key card in your hand, it isn't your room. Way too many variables to be able to say, with any certainty, that a particular room is yours until you have checked in.