I got pulled over on I-39 near Janesville on a Friday morning as I was going to pick my dad up for his chemo appointment. I wasn't speeding so the only reason they pulled me over was the front plate. I think one cop may have been training the other because two came to my car, which seemed unnecessary. I was just told to put my front plate on and they let me go. That was the only time I've ever been stopped for it.
When I lived in Chicago, I got a ticket for no front plate while the car was parked on the street. They don't seem to be that aggressive here in Madison/Wisconsin.