I've had NRA stickers, my Navy Alumni Assoc. sticker and USN on my small, rear side windows in the past. Not surprisingly, the only comment I received on any of them was from a woman I worked with who questioned why I belonged to the NRA. I answered her and she was satisfied. Even here in the Northeast, you see a lot of pro-2nd Amendment stuff on vehicles.
Like M58 mentioned, I don't like anything on the vehicle other than the windows as it's much easier to remove when you get ready to sell or trade. Today, my new car has nothing, not even the dealer crap!