Got to agree with Dub. Most recievers attatch in some way, shape or form to the frame. Jumping on your bumper is not the same as 200lbs hanging out in front (not dirrectly over, which means in reallity more weight and stress) combined with road vibrations/bumps. Better safe than sorry...
The issue that I would be concerned about is winching off-angle. A lateral load on the winch or even a tow point could deform the metal if it isn't braced adequately.