In the flat we just moved out of, the lease stipulated no hanging without permission of the landlord, but i put up 2 or 3 pictures anyway, and when I moved out, to my surprise, the leasing agent, rather than asking us to fill the holes, asked us to leave up the hooks for the next tenants to use! We did have £20 deducted from our deposit for one missing light bulb and for not leaving the shower door clean enough (we tried!).
In the house we just moved into, we happen to have a young, very easygoing landlord who is letting us hang things where ever we want. Hopefully he'll also be easygoing when we move out and it's time to get the deposit back!
Ron