At least for the first few waves of migration, I try to make sure every dwarf has his or her own bedroom. So each one gets a small room with a bed in it assigned to him or her. But the dwarves don't seem to take any notice of this: they all just sleep in whichever room they feel like!
Now, if this doesn't bother the owner of the room (I don't know whether it does) then I might as well just leave all the rooms unassigned and let the dwarves sleep wherever (this would save me a lot of room-assigning whenever a big wave arrives) and I probably wouldn't need as many rooms, since not everybody wants to sleep at the same time.
Is there a way I can force the dwarves to sleep only in their designated room or in the overspill/new migrant dormitory?