I had a sticker plate for awhile but it was spotted one night when stopped at a booze bus, was told to remove it. If caught with it again I would face a court appearance.
So I did.
However I didnt put my proper plate back on (since I had got rid of the hideous looking Evo mount), I just sat it on the dashboard. Got caught by an undercover cop after a few months and fined $50 for not displaying a plate.
Up to you if its worth the risk.