Most of the time, I deal with a dealer who's got mad cash and mad weed. He doesn't need a tip, and he doesn't need anymore weed. Sometimes I'll smoke a joint with him but that's because he oftentimes smokes a blunt of his shit with me for buying from him.

A good dealer shouldn't expect a tip. If you have a smaller time dealer who just sells weed and can't buy it or whatever, I could understand tipping a little nug or so.