I've purposely squared knife spines on belt sander, both at right angle and canted. If spine is truly sharp, it didn't matter what angle is it at, it'll scrape firesteel just as well.

I've only seen this in two knives I have, a Mora craftline allround which spine is factory grounded, and a SOG NW ranger that I can't tell since it's black coated. Never seen it in a folder.