I've got a bunch of 'em on my truck, mostly political (but also some for various charities I support), and generally for low budget insurgent candidates for whom bumper stickers can be a cheap means of exposure.
I'm not worried if someone takes exception, because 1) my Silverado is older than most of the candidates- so go ahead, scratch it, dent it, see if I care. Hell, go ahead and steal it, it will give me justification for buying a new truck :), and 2) if I'm in the truck and someone takes exception, I'm perfectly willing to show them that my "I support the 2nd amendment" bumper stickers are more than just a slogan (I have a little friend tucked next to the seat- for those who watch the Colbert Report, my friend is very similar to his friend "Sweetness").
I still have some "Ron Paul 2008" and "Ron Paul 2012" bumper stickers on it. I'd take them off, but I'm worried that they are the only thing holding the truck together!