In NM the Law says it must be legible and visible. "The plate shall be securely fastened at all times in a fixed horizontal position at a height of not less than twelve inches from the ground, measuring from the bottom of the plate. It shall be in a place and position so as to be clearly visible, and it shall be maintained free from foreign material and in a condition to be clearly legible"
It does not say from where it must be visible and I have never heard of anyone with a trailer being stopped for having the plate obscured by what ever is in tow. When my 5er is behind the truck you cannot see the truck plate unless you hunker down under the overhang, but from there it is visible. ;)