I never break the spines of my books. You can look through books I've read 100 times and the only sign you'd have that they weren't new is the unavoidable yellowing creeping on the pages.
My best friend bends the spine back before she's even read a word. Claims it's easier to read (which I kind of understand, I can't hold a book one handed when reading and not spine breaking) and also makes her books look 'loved'. I personally think if you loved it, you'd look after it.
She isn't actually allowed to borrow any of my books for this exact reason.
Where do you fall on this debate?
Have you avoided lending someone a book because you're worried about the condition it'll come back in?