I have one sticker in my back window. It's a company logo (Black Diamond Equipment) that I have a ton of gear from. (They are probably THE biggest manufacturer of climbing equipment) Very small, maybe 1.5" X 4" I refuse to put anything political on my vehicle since that just makes you a target.
On a side note, I did see a really cool and tasteful one. Just small lettering on the back window of a car in front of me that read "TEA Party on Dude."
Indeed, the political stuff makes you a target.
Funny story. When I was in college I was very active in the pro-choice movement. I had two pro-choice stickers on my truck. One was a simple no coat hangers one, the other read "I'm pro-choice and I shoot back" (a response tothe clinic bombings that were going on in the '90s). Anyway, I was living in a really sweet Victorian house, who's owner's paid the mortgage by taking on a few room mates. One was a retired Army sergeant who was the least political guy I've ever met. He asked to borrow my truck to go the grocery store. I gave him the keys. He came back to find himself parked in. He went to every store in the five store strip mall and asked that they announce that the owner of the car come and move it. He waited. Fifteen minutes later, this woman (think the Church Lady from SNL) said that it was her car that had blocked him in and that she'd only move her car when he took the baby killing stickers off "his" truck.

He tried to explain that he'd just borrowed it, and only wanted to go home. Her response? "I see you've been to the liquor store. One sin begets another". He walked away and called the cops. When they arrived, she'd just finished ripping one sticker off and dropped it on the ground. She got ticked for littering and her car was towed.
Even at 21, I learned that sometimes its just not worth it. I'm supporting a cause, not looking to pick a fight. I have been sticker free since then. I only wear the colors when I want to make a point. Then I wear them proudly (NRA, pro-choice, anti-drug war etc.). When I just want to go to the store? I leave that stuff at home. YMMV.
FQ13