For the sake of learning,why shouldn't the barrel ever rest on anything? A safety issue or an accuracy issue?
Strictly about accuracy. Resting the barrel on anything changes how it moves when fired, thereby changing point of impact.
I have used shooting sticks for years and never had a scuff mark on my wood stocks. However if your sticks don't have rubber padding of some kind it could scuff the finish. The rubber helps the sticks move with the rifle during recoil.