Here's what I think: Bumper Stickers
No matter what you may think when you buy a bumper sticker, that thing ain't coming off nicely. It will take a whole lot of effort to get the sticky residue off your polished bumper. A beat up, rusted out, crap wagon (similar to the 1980 Ford Fiesta that I was honored to drive for my first car) can benefit from a good bumper sticker collage, but why spend that kind of money on a Lexus (a luxury car) only to ruin the luxury finish?
It may seem like a good idea to plaster your opinions for all to read, but think about those poor fools who are stuck with the George W. Bush campaign stickers still today. I'm sure back in 2000 it seemed like no harm done, but eight - even four - years later regardless of whether you still supported the man, that bumper sticker probably didn't seem like such a great thing. I'm all for supporting candidates and presidents, but maybe do it with a window cling that comes off clean when you change your mind or the theme is no longer timely.
I'm just saying use your best judgment, if you think the sticker will outlast the longevity of a car (for some 20+ years- unless you're hoping to trade up within a few years) then by all means bumper sticker it up! But just in case you don't want to wreck your shiny paint job - maybe look to posting your opinions for all to see on facebook or twitter or whatever the newest and best social network is, you'll likely reach more people that way anyway.