For many years I did as the OP does. Some older prints show discoloration of white mats around the edge of the print. This is also true of a Cole Weston print from about 34 years ago. Now I print with a white (or occasionally toned) border that extends beyond the overmat window. Smaller prints in 12x16 frames are trimmed to that size and are self-centered in the frame. Larger prints are hinged to a backing board.