I think a flat tag would be best. When I worked the road as a trooper, I'd stop folks for plates leaned up in the rear window of their car (sometimes held there by a Kleenex box). I'd never write a ticket for it. I'd educate them on getting it fixed. It did allow me to legally stop them though since it didn't meet Secretary of State Vehicle Code requirements.
It was usually other things (guns, open alcohol, loud exhaust, seat belt, drugs, drunk driving, etc.) that I would find more interesting or necessary to take enforcement action on. I had many traffic stops that were had over license plate display violations that never received a ticket but opened the door to meet a trooper.