@Sapphir567 Well, you can't assign rooms. You CAN assign beds though. You can also do as
@PLN1993 mentioned and lock a room's door for everyone but a certain sim. I've never locked individual rooms unless it's a room I don't want townies going into.
Most of the time just assigning the bed is good enough.
However, if you're role playing and it's an "apartment" style build, Yea, I would lock the doors for everyone but the sims you want living in that "apartment". You might want to assign the bed also. JIC.