I have on my car my first political bumper sticker ever - an Obama '08 sticker that was placed there shortly before the North Carolina primaries (where I live). What’s the etiquette for bumper stickers after the election? Does it matter if my candidate wins? I’m still occasionally seeing Bush/Cheney and Kerry/Edwards stickers. What’s the vibe?
I don’t think there’s any particular rules, though those that support a losing ticket probably remove them fairly quickly. (Those that support the winners keep them on to gloat. 
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Etiquette? I don’t think there is such a thing as bumper sticker etiquette.
A certain percentage of people leave stickers on after the elections. Some, like my wife with her years-old Elizabeth Dole sticker, are maintaining a reminder of a candidacy that was notable in some way. Others, like people who keep stickers that I personally don’t care for, are pathetic losers who don’t know when to move on. 
I guess if (when?) Obama wins, I’ll just keep mine on, then! I’ve heard people, here and there, comment that if he wins, it’s toolish to keep the stickers on. Gloating, bad form, and all that.
I wouldn’t consider it gloating; more like a show of continued support.
My car was in an accident recently, and since it took a while for the insurance details to be ironed out, it won’t be repaired until after election day. My Obama sticker arrived the day after the accident. If he wins, I will put the sticker on the car when I get it back.
I still see a lot of W in 04 stickers around. And the very occasional Kerry 04 sticker. There’re no rules here; do what you want.
If Obama wins, I bet there will be a lot of Republicans who suddenly decide that an Obama campaign sticker is gloating & socially unacceptable.
My personal favorite is one I saw in the parking lot at a prior office. It appeared during the 2004 campaign & said “Re-defeat Bush in 04”. It was still there when we moved out in 2007. I hope it lasts many more years.
I’m keeping my “LGBT for Obama” sticker on (assuming he wins). Is this gloating? Sure. I haven’t had anything to gloat about since '96.
The other day at the garage I saw a car (no longer functional) with an extremely faded and rusty “Citizens for Reagan in '80” sticker on it. Now that’s dedication.
I’m peeling mine off after the election. I can’t necessarily guarantee the morning after, as I’ll probably be tired and I’m a lazy sod, anyway, but certainly within a couple of days.
I’ll leave mine on even if my candidate loses, just to say, “see, the hell you have unleashed upon us is not MY fault.” Seriously.