Let's talk about bumper stickers. I hate them. Sure sometimes they can have mildly amusing sayings. In fact I've often thought about having one made that says "Genius is the cross I bear." But I still hate them.
I just don't understand why you'd put stickers on an expensive piece of machinery that, more than likely, you will have to try to convince someone else to buy at some point. I have used a bumper sticker on my car for one reason--to cover up a stupid mistake I made while learning Lori's Life Lesson #378: "Just because there was no one in the driveway behind your car when you parked it last night, doesn't mean there is no one behind it this morning. Defrost your back window."
No one cares where you like to go on vacation, where you had your cat neutered, or what your "other vehicle" is.
Things you stick on your car can fall into various categories:
Stickers with dates on them: They're really only "cool" the year you got them. Who cares that you went to Marriage Encounter 2003? It's 2006 and you're divorced.
A large subcategory of stickers with dates are campaign stickers: You lost. Now what?
Stickers proudly proclaiming your religion: You'd better represent then. No road rage, flipping the bird, or cutting people off in traffic.
Stickers with punny sayings: Is it really that funny? Is it funny more than once? Do you really need a conversation starter on your bumper?
Stickers broadcasting your sexual orientation or your opinion on any sexual orientation: I like my tires full and my car un-keyed, how about you?
Magnetic ribbons: Enough already, huh? The only thing you are supporting is the car magnet manufacturing industry.
So resist the urge. I'm still going to laugh, even if it's paid for.