There are plenty of bad drivers out there (perhaps the majority). Those individuals who advertise thier offensive personalities are just more identifiable. I think that it is a reasonable deduction that someone who chose to put an offensive advertisement on thier vehicle "My bully just beatup your honor roll snot" etc. not someone who will go out of their way to "drive polite". OTOH, you can't deduce that someone will not ram you for being slow when the light turns green just because they aren't displaying a bumber sticker.

My personal experience is limited since I seek anonymity in public. I once bought a car with a marines sticker in the window and it helped in getting some forgiveness from the PD. Not that I lied but I did nothing to correct thier presumptions. Same with the "Troopers are your best protection" stickers. Even though I personally believe that my friends "Mr. Smith and Mr. Wesson" are probably my best protection.

No matter what your bumper sticker says it will cause offense to someone. Driving through a jewish neighborhood with a cross on your bumber will offend someone, Driving through a Baptist neighborhood with a Star of David on your bumber will offend someone, etc... What you believe and say to individuals will be taken in the context of your relationship with those individuals. What you braodcast from your bumber will only be taken in the context of your driving behavior as displayed at that moment. If you hold a view strongly enough to want to advertise it and perhaps engage in unnecessary fights with anonymous individuals whom you would not otherwise meet over it then by all means advertise that view as loudly as you can to as many as possible.

OTOH, if you think that your driving habits may reflect badly on a group that you are trying to affiliate yoursef with then you might want to reconsider attaching thier logos onto your vehicle. There are many very lousy drivers on the road whose bumberstickers proclaim that you can follow them to heaven but their driving might convince you that they are truely in a hurry to get there.