I've learned to be very picky about the people I'll smoke with. If someone makes me uncomfortable for any reason, getting stoned with them or being around them when when I'm stoned is pure nerve-racking hell. This is not MDMA or alcohol we're talking about here. Marijuana doesn't, I find, have the power to 'smooth over' interpersonal differences. On the contrary, I find it tends to bring them sharply into focus.
Just off the top of my head, I'll generally not smoke with:
* People who give me the impression they like to fuck with people's heads for fun
* People with mean-spirited senses of humor
* People whose opinion of me very much matters to my life or livelihood
* People who like to have pissing contests with me, or are looking for ways to feel better than me
* PEOPLE WHO LIKE TO ARGUE, AND TAKE IT VERY SERIOUSLY
When I get stoned, I like to feel free to just talk and laugh about ridiculous shit and just be myself. I shouldn't have to defend my honor or keep someone from trying to get the better of me, and I sure as hell won't be someone else's entertainment for the night, except for my wife. I want to feel free not to talk much at all, if the high doesn't make me feel like it.