I've got my NRA and GOA stickers on the drivers side front windshield of my jeep, and no gun affiliated stickers on the rear of the vehicle. Since we have CCW and open carry here in Nevada, I can't imagine that the stickers would be anything other than a deterrent. I mean what criminal, other than a terminally stupid one, would break into a gun owners car knowing full well that the owner is probably armed and could surprise him in the commission of the B&E? That's just asking to get shot.
The vehicle is garaged and all the toys locked in a safe when we're not home, so I'm not too worried about someone breaking in and stealing my little collection.
As an aside, I have a sticker that says 'God, please protect me from your followers.' on the bumper that I've had since long before 9/11, and ever since that date I've gotten several people who want one like it or just honk and give me a thumbs up that they approve... It's sad, but appropriate to what's going on these days.