My rule of thumb in Boston is that if you do something people find heinously annoying, rude, or morally unacceptable, 99% of people won't do anything because they're too polite. But that 1% will do something, and that something will be a day-ruiner--yelling and swearing at you, destroying your property, or occasionally violence. There is a not-insignificant portion of people in Boston-Cambridge, especially younger people, for whom seeing this license plate will not engender good feelings toward you.

Personally, I think it is likely that if you get this someone in Boston or Cambridge will eventually key your car. It might not happen right away, a year or more could go by, but probability will array itself to find that 1% eventually.