Absolutely not. Even if the book is old and tatty I won't do it (Would you believe I feel guilty about it xD).

When I was younger I actually went through a stage of collecting and making my own bookmarks so I normally have one on hand if needed. If I don't then I use a piece of paper.

I don't like folding my textbooks, especially if they're new/newish ones (I like that clean, just bought look :3 ) So, if I need to bookmark a certain page for a reference or whatever, then I'll use a post-it note - Certainly saves ruining the book :P

xD But thats just my opinion