I've had several vanity plates and haven't had an issue. For the M5 I have "PFFT AMG" coming. My car friends (several of whom drive AMG cars) get a kick out of it, partly because we make fun of some of the slightly less intelligent/sensible idiots who drive them and always want to street race.
I guess it just depends on where you are.
2014 X5 xDrive50i ZMM - Alpine White/Mocha (Mods In Progress!) - 2013 Cirrus SR22T
2014 Jeep SRT - 2013 Tesla Model S P85+ - Bimmerpost Featured
2013 F10 M5 - 2012 550 xDrive - 2011 M3 ZCP