I don't have gun-related stickers on my car, because to an anti who doesn't know me, it just serves to reinforce their stereotype of "all the gun nuts out there".
Likewise, I wouldn't wear an NRA lapel pin to a job interview, because I don't see the point of making my shooting/carrying practices the very first thing someone knows about me. I'm also a photographer, motorcycle rider and naval officer, but I don't wear a helmet, carry an SLR and have my ID card on a lanyard around my neck 24/7 either.
Plus, as mentioned before, there's the whole theft aspect. I had a very expensive stereo setup stolen out of my car once, and I'm pretty sure the big "Precision Power" and "Linear Power" amp manufacturer decals had something to do with it. While I don't leave guns unattended in my car, I'd have to have a thief break in "just to check".