The state statutes also specify that it has to be illuminated at night, unobstructed from a certain field of view, and "clearly visible" at a certain distance.
Had some dickhead deputy pull me over in my truck because my aftermarket bumper "partially obscured" the bottom of my tag... and it's been that way since I bought the truck new in 1994! The cop was maybe 5'2", so that's probably why he couldn't see over my bumper. The point is, just because you get away with it for years, doesn't mean you always will. So now when I get a new tag for the truck, I drill new mounting holes that position it 2" higher.