I know for me, there are situations where I am OK with someone drafting off me, if I don't know them. Although, most of the time they just make me nervous.

At my last rally I had this guy drafting off to the left of me due a side wind. This guy really got in my nerves. I was weaving at times, and he was sitting off my wheel and overlapping our wheels. I was scared of weaving and his wheel hitting mine and both of us going down. He finally moved on and did it to someone else ahead of me.

Otherwise, I don't draft off strangers. I don't trust them enough.