Here in Ohio it is mandatory..... and I "forced the hand of the LAW" on more than one occassion. Each time, I would tell them that I asked them there at the BMV (DMV etc... for some) when I got plates, and they said the only thing they could state for certain was that "they strongly encouraged" placement of the plate, but did not "think" it was required.... (yeah, RIGHT!

) I stated the same to several officers over many years who similarly did not produce any specific "evidence", but told me to "put it on" as soon as I got home, etc...
Then one day, a mature "full grown bear" (we have a lot of those in Ohio ) pulled me over for speed, and calmly stated the license plate was missing and required immediate attention. (He let me off on a warning on the speed... well over 80 in my Supra...) I told him I had never had the plate on, because of the cosmetics, and pointed out that there were no "holes" in the bumper....(true), and explained the "project car" nature of the Supra.... I explained the "BMV position", and did it politely. The mature officer proceeded to the trunk of his trooper, pulled out one of two 5-6" thick bound volumes, and sifted through it until he was able to cite "....motor vehicle license plate display standard blank-blank-blank.... section x, sub-point blah-blah.... and there is was in black-and-white. He looked me straight in the eye and said, "well, that clears that up for you now.... I would advise that you attach your plate at your first opportunity, next time, I would have to cite you."
I thanked the officer, and committed to replace the plate. I also replaced my other vehicle front plates.... until the Solara came along, and the dealer drilled two self-tapping screws into the bumper the day I took delivery. Didn't even fit the CONTOUR of the bumper.... YUCK! Furtunately, as luck would have it, I was in a rush and went through a reputable soft-cloth automatic wash.... and when I did my regular "walk-around" after the exit.... the plate was "hanging on" by just one screw, as the other one had "pulled through" since there was no washer on the undersized head of the screw.
Now my plate sits in the trunk, bent with a "countersunk" hole from the "pull-through". I leave one screw in the bumper hole, and the other an open hole. Both are quite small, and you really don't even see it until you are RIGHT on top of the car, and even then, it tends to escape the eye. But IF I should ever get stopped, I have my 100% guarenteed alibi for Officer "SIR", on any given stop.
- crisp