I don't think it matters with 'slanting' a chop, but if your tool tends to tear the bark from the bole, go the other way so that the rotation of the bit is inward, toward the center of the cut or use a different bit. I use 'structured tooth' carbide bits in a Dremmel tool that make a fairly smooth cut. If I've erred and made a ragged edge, I finish by making a clean cut of the exposed cambium with a grafting knife. In the case of dieback of a branch, though, I try not to damage the branch collar other than incidental knicking of the inner ring. Again, accidents can and do happen and I just clean up the ragged edge with a grafting knife in the end.