Sebastion points out that covering your car in NRA bumperstickers isn't a good idea. Guns are one of the major targets of property crime. Criminals want them and so there will always be a black market for firearms. Criminals especially like to know where you store them, because they don't want to rob an armed man or woman, but they love to rob unoccupied houses or vehicles.
When you put an NRA, pro-2nd, or pro-carry sticker on your car, what you're saying to criminals is "this car might contain guns, rob me when nobody's around." This is also true for anything on the outside of your home.
In a slightly related concept, you're not really concealed carrying if you're wearing overtly pro-gun gear. Some of the esoteric logos might be fine, but anything else calls attention to yourself and makes people take a second look. Not a good idea.