I would never put a bumper sticker on my car. I just don't think it's necessary to promote my favorite radio stations--or any stores or causes, for that matter--that way.
Whenever I see a bumper sticker for KDHX or WSIE, however, I think, "That must be one intelligent driver with good musical taste."
I would also think the same of anyone with a K297BI Classic 107.3 bumper sticker, but I have yet to see anyone driving around with one. I wonder if their listeners feel the way I do about bumper stickers.