Also, just because you TOUCHED a ring barely enough to mark it doesn't mean it's ruined.  Obviously, you don't want to  cut deeply into it, esp across.  And if you can see the next ring down, , or knocked down a knot, you messed up.  But hovering reverently over the winter ring above the one you want and  rubbing laboriously down to it with 280 grit paper isn't necessary.
 A tiny, barely visible, longitudinal tool mark simply might not be a problem, where a transverse nick or gouge likely would.