I wouldn't phrase it as "heresy", as I won't get on people's cases about doing it to their own books, but it always makes me cringe and if someone did it to one of my books, I'd beat them with a stick. That being said, I do sometimes add small bookmarks or (in sturdier books) small stickies to pages that have passages I particularly enjoy or want to remember. I used to dog ear, but I got out of that habit.

In books that I'm reading for academic purposes, I will sometimes make small marks in the book. Always lightly, and I will usually try to use some other form of marking first, though. I never highlight anything except textbooks. *shudder*