I avoid bumper stickers for a few reasons. Primarily they're an eye sore, just like the campaign signs that are stuck in every lawn and street corner during election season.
Second, I don't feel it's an effective way to influence others' opinions. The drivers behind me don't care what I believe in, they just want to get from point A to point B.
Some stickers, such as "Don't blame me, I voted for Kerry" are downright childish. Most of those can be found on cars belonging to union workers. It's funny how some of them revere the union life almost as a religion, but that's a topic for another thread.