I have on occasion run across lenses offered at really good prices that have cleaning marks/scratches/abrasions on the front or rear lens surfaces.
For the most part the ones I grabbed had problems with front surfaces but that had to be pretty bad before it was noticed on the neg, usually in the form of flare/ghosts.
My question is: Damage to which surface will be more readily seen on negs, front element or rear element? Or does it really matter?
I once purchased a dropped 50/2 Summicron for $5 and black lacquered the moon shaped crack on the edge of the front element and used it for many years with no noticeable effect other than it was probably an f2.2 lens. Ugly as all get out but print for print it matched just about anything out there!