It's a tricky one because most insurance companies have conditions that you must inform them of anything that may alter your risk profile as your premiums are based on your risk profile.
If you have a very minor accident and the damage is less than the excess or the other person was at fault and you have it repaired under their insurance, you still need to tell your insurance as it may have a material effect on your risk profile. Failure to do so could result in a future claim being rejected on the basis of misrepresenting your risk profile.
For small scratches, etc, I really don't think it should affect your risk profile, but rather be safe than sorry and inform the insurance. Down the line, the insurer could just see that your bumper was sprayed - there is not proof that it was just small scratches and they could assume it was to cover minor accident damage.