What type of strings are bad for the frets?
Whenever there is metal rubbing on metal, there is friction. When there is friction wear is going to eventually occur. Various string profiles cause different wear patterns. Round wound strings can dig into a fret a little quicker since the pressure is concentrated on the tangent of the winding. It is like taking a blunt needle and rubbing up and down on a piece of metal (or fret). A groove is eventually going to occur where the winding is located. With flat wound strings, the pressure is more evenly distributed across the winding as the winding is flat.
