Considering the bumpers are held on by 4 bolts a piece, I think you're safe going on whatever roads you like, without fear of them coming off (unless you're like me and the Nissan Dealership sold you a Micra where the rear bumper was held on by hopes and dreams) - but if you're really looking to give it a rumble then motorways are actually the worst place too smooth - go for some really rough back roads, went down a particularly bad one too quickly and managed to rattle my entire car stereo wiring out, and had to go the rest of the journey in silence
The more likely candidates for motorway casualties are wipers (lost three in a year and a half to the M40 and M25), windscreens (for when that lorry in front of you fires the odd stone up), and generally anything loose on your car (such as an L plate on the dual carriage way at 70).