I have touched up the crown on a few of my Milsurp rifles. To me, much of my decision depends on the type of rifle, how collectable it is, and the type of crown. Some types of crowning can be touched up with no real tell tale signs of having been done, while others are quite obvious.
Counter boring was often done for a different reason...that is bell mouthing the muzzle from improper cleaning with steel cleaning rods. I've seen this condition more on Enfield's and Mosin's than any other rifles, although I sometimes find it on U.S. Military rifles. Enfield's in particular, really need careful scrutiny and the "bullet in the muzzle test" before purchase. Same goes for P14's.