Wow, people here really hate bumper stickers. I think of them as a fun little bit of self expression that reminds me there are other humans in the road. I really don’t think the risk of getting your car keyed because of one is nearly as high as it’s being made out to be, and police using a bumper sticker as any sort of evidence seems pretty unlikely too. Do whatever you’re comfortable with of course, but I wouldn’t be too worried.