I like to have a clean presentation of my work. Sure that outer edge of the mat might be hidden under a frame eventually, but that’s not what a customer sees. They see the matted print.
Sometimes when cutting a picture mat the outer edge gets fuzz or looks jagged. Sometimes I also find this on the cut mats I order. To clean up the outer edge (not the mat window edge but the outer dimension) I’ll take a piece of somewhat fine grit sandpaper and, avoiding the mat surface, I gently knock off the fuzz on the cut edge. It works for me most of the time but some boards just don’t clean up well.