a scratch on the rear element is usually pretty bad. if you're wondering whether or not you should buy a lens with such a scratch, I would be very hesitant. a small scratch on the front element, however, is usually quite harmless, especially with telephotos, and many deals can be found for lenses with such scratches. if you don't believe me take a long telephoto at 200mm or even more and put your whole finger right in front of the lens and see how much of a difference you see. that's a whole finger. now just imagine a piece of thread. that's like a huge scratch across the entire lens. so what about a puny piece of lint instead of a small scratch? you won't be able to see that even on shorter focal lengths stopped down to smaller apertures. anyways, I digress. if this is your lens with a rear element scratch, do tests. if you're buying, then do some test shots in store or whatever. vary the focal lengths and apertures. pay special attention to shorter lengths and smaller apertures. if you can't do tests (ebay), then don't bother unless it's practically free and wouldn't mind losing the money.
Gavin