This is a First Amendment issue already addressed by the SCOTUS back in 1971 in the case of Cohen v. California.
Cohen v. California
This woman signed an agreement not to participate in any "partisan" activities because of her job.
If the sticker said F**K W's War then it would be a violation of th that agreement.
I think we can all agree that, no matter how necessary a war might be, war in and of itself is a horrible thing. So to war I say f**k that.
As far as people "seeing" it and being offended I say f**k them. Getting offended and makinga big deal about it has made more noise than that little bumper sticker ever could on its own.
As I've explained to my daughter, she's 5 and a half and is still figuring things out, words that you hear or read ONLY have the amount of power you give them NOT the power the person saying or writing want them to have.
She knows that there are words that if she says them are given enough power given by people at school that she will get in trouble. So I've told her to use them judiciously and only when needed.
If you aren't willing to take 8 minutes and talk to you kid about words and how you want them to deal with those words they are in for a rude awakening later on. It's like giving a kid a sip of beer once in a while when they are young. As they get older and their friends are fascinated by booze your kid's not impressed because it hold no mystery for them.
Anyway I think the Court case pretty much says it all.