I did once when I cancelled insurance and registration but kept the car it was attached to. Everything I read from PennDOT at the time was unclear, but it seemed like they didn't really care what you did with the plate, unless you were cancelling insurance or registration, in which case they really wanted the plate back. Post office said it would only cost me a couple bucks and some change to send it in, I figured why not, only a couple bucks to prevent potential headaches.