Most Helpful Girl
It kind of depends on the moment and how comfortable I am with him. A guy I've just met, probably not the best idea to play with my hair yet. But, when the guy I like now, who I've known forever, was playing with my hair I loved it. I guess it's kind of a trust thing- I'm letting you in pretty close and I don't really let many people besides my hair stylist and my sister play with my hair. Maybe test the waters by reaching for her hair, and if she pulls away just say she has a fuzzy or something in her hair- pull the imaginary fuzzy away and return to flirting. No awkwardness and you know that she's not receptive to you touching her hair.
Also, you have to remember it depends on the situation as well. If you're at a wedding and the girl has just spent 2 hours on her hair, she probably does NOT want you playing with no matter how close you two are. But, if you're just hanging out on a normal day, then it's not so off limits. Of course, this is coming from a girl who rarely spends more than ten minutes styling her hair on any given day so maybe it depends on the girl too.