I've recieved a few here and there, hell I even got $20 from a discovery flight before it even started. Some people are completely against it, but honestly you are providing a service. If someone feels that you went above and beyound your duties, or just how you handle any given situation, then it is a reward and acknowledgement of that service and attitude. I'm always flattered and appreciative when someone offers, and usually I'll mention that they really don't need to, but I do appreciate the gesture.