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.
If it is a very minor scratch, it should not affect the pics at all. I have lenses that are scratched up on the rear elements and they still perform pretty ok. Pics are still sharp
I even have lenses (standard 50mm lens) that are fungus contaminated that still turn out good, sharp pics. The only thing that I have found is that there are tendency to flare depending on the lighting conditions. But I still have not given up on the lens as it can still performs. You just need to know the conditions that will cause it to flare and avoid shooting it those conditions.
By all means take some test shots if you can. Don't worry too much about minor imperfections of the lens. Most lens even new brand new lens are not perfect. Next time take a small xenon flashlight, go flash through a brand new lens, you will get a shock how much dust particles and even slight haze you will find in the lens - even leica M-series lenses. But they will NOT affect the pics in at all.