i buy a lot of peds from Mass and dont get to registering them for a while. the sticker will still be good, but no NH regie. ive never been stopped.
shit, now i think of it.. every moped i buy i ride for at least 2-3 weeks without a plate.. i never get pulled over. i dont think you should have a problem. but if it _is_ registered, then you might as well bring your registration info with you just to prove it.