I think you're right, Dr. Word. I've always heard the expression "mind your P's and Q's came from pirates/rowdy types who frequented bars about 200 years ago. They frequently became extremely drunk, so bar owners would take advantage of them and tell them they bought more beer than they did (ie pints/quarts), so they would be told by the captain/other person (who had to pay for it) to mind their p's and q's, as in pints and quarts.