I put the "bumper" stickers on the body.
My last car (a Ford Aspire) only had a fairly narrow strip of metal on the back, what with windows, tail-lights, lift handle, license plate depression, etc.
My current car (a PT Cruiser) has all the room in the world for magnetic stickers.
But where is the metal for it to stick? These things are covered in plastic and fiberglass, and separated from serious metal by a gap of air.