I don't even remember where I have learned that, but I've always thought it common knowledge: Before a piece of meat gets seasoned, or malleted for tenderizing (sorry, don't know the English word for that), or marinated, or thrown into the pan, or ground, it should be first washed under the faucet, and then patted dry. When I think of it, it also makes sense to me, because the bacteria are always on the meat surface, never inside. So I don't think it is a vestige of the time one got bloodied cuts from the village butcher, but that it applies to the modern meat pieces sold on absorbent pads too.
And then I read this question, which presumes patting (but doesn't mention washing). Most of the answers and comments seem to indicate that patting isn't always considered necessary, and there is no mention of washing the meat or of the dripping water from the washing. In fact, the answers and comments wouldn't make much sense if one assumes that the meat has been just washed.
So is washing compulsory, is it optional but a good idea, is it plain useless, or does it even have disadvantages for the meat?