It isn't the bike that cops will object to but ANYTHING that obscures the tag will get the attention of 'The Man'. They want, need that identification and might give you trouble if they can't see it. That is the case in every state. Like everything else, some cops will ignore it but there is always the guy with a chip on his shoulder who will pull you over for the slightest infraction. Just be aware that it is illegal in all states to hide the license plate of a motor vehicle.
Tom