At the Ren Faire we play, the deal with management is that we play for what they pay, plus any tips. We originally had out a basket with our brochures and business cards, and people kept putting in money, which, if it was paper, would blow away--now we put out a pewter tankard. We don't mention it, but an amazing number of people put in money.
Two years ago, we were playing a street gig, paid by the town's parks 'n' recreation department, and a couple of passersby opened Reynaud's concertina case to put money in. They spoke with us when we finished the tune, and said we should leave a case open. We replied that we were getting paid. Their response was along the lines of "So what?"
Now, depending on the venue and the "feel" we'll leave out the tankard or a basket, but we don't mention it. It beats having someone ask me in the middle of a whistle reel "Where's your tip jar?"
I guess my thought is, if the audience is looking for it, give it to 'em!
Just my $0.02 fwiw, your mileage may vary.