There's nothing wrong with one or two bumper stickers. If I had to generalize based on my own experience, though, I'd note that lefties are far more likely to have a car slathered with bumper stickers than those of us on the right. That's simply because they want to force their views on you, whether you want them or not. At least they're consistent, though--they want to force the government on you as well, but only if they're in charge of the government.
Here's an SF Bay Area (hint: leans left) news report about a small link between bumper stickers and road rage.