A tip jar seems in poor taste to me. In fact, it might cause more problems than it's worth. As an occasional GM of public games at my local FLGS, the biggest problem I encounter is players who show up with a chip on their shoulder about entitlement. If someone feels entitled to act like a jerk without any incentive, how much worse would it be if they had the additional attitude of "I payed my dues, so I can act however I want."? Not a good idea, IMO.