I have scratched the underside of the front bumper two or three times. All occurences were during the first year or so of buying my C5.
The problem for me was that I was very cautious when parking facing a curb or concrete stop. But sometimes I misjudged how much of my rear end was sticking out. I therefore tried to pull forward too far.
After about a year I learned from the scratches that, as you said, don't show but we know are there. I have now -- I hope -- learned. When I am not certain how close to pull my rear end up, I just stop, put it in neutral, set the hand brake, get out, and look before I move another inch forward.
These extra steps take a few minutes at most and prevent years of discomfort.
Now that we have learned, it's not really so bad if the total cost is limited to only a few, nearly invisible scratches.