Euro plates in the US are fine if you want it for decoration, but are not a legal substitute for a real US plate. And in general, if you have one on in a state that requires both plates, it just screams "hey I'm not listening to the law".
At least with no front plates the cop might just think you're from out of state unless he turns his head and sees the real plate on the back.
Current: '15 M235i Vert
Previous: '13 335i, '08 TL-S, '00 Corvette