My parents had a rule about me using the walls to hang stuff in my room, but they kinda gave up enforcing it when I got a bunch of Beatle stuff.
I have an aunt who is ridiculously strict about her kids putting stuff up on walls. They're not allowed to have posters or anything, it's depressing.
My teenage bedroom walls, though....it's like an explosion of postcards and posters. Completely mad! My parents were repainting our interiors and had to skip my room completely because they couldn't handle the work of pulling down all my pics. Muhahaha!