I gave my plate a quick furtive lick once at work, and manager happened to be passing the kitchen at that moment and totally called me out. Ever since i will not lick plate.
If the utensils can't pick it up or the bread can't, and if it's good enough to get fingers sticky, I'll lick a finger.
It goes back to ancient times if you bend down to lap water like a dog or stay alert and cup a palm of water to your mouth.
We have to stay alert and not lick plates.