It may depend on the lens and the scratches. A small scratch on the front lens may pick up light outside the frame and cause visible artifacts.
A significant scratch on a rear element that is close to the sensor could cause a visible disturbance. The rear lens is usually smaller so scratches may cover a larger relative area.
My guess is that small scratches that are hardly visible would probably not cause any problem.
I was told recently, by a salesman who I felt would know and who wasn't influencing a decision on my part (I was looking at new lenses) that scratches on a rear lens element are much more important than scratches on a front element. So I thought I would see some mention in this thread of that distinction. Is that not the case? --Barbara