I have three super close friends, and only they are constantly allowed to touch my hair. If anyone else does it I pretty much tell them to, "Stop touching my hair or I'll punch them so hard they won't come near me again", in a very serious I'll-kill-you voice. Whoever touches it, it feels extremely weird 'cause I can't really pin-point where their hand is since my hair goes down to my buttocks. xD
The only people I let touch my hair are people whom I am close to and am generally relaxed around. I dislike being in close proximity to most people, so if I allow someone to get close to me it generally means I am comfortable enough with them that they can basically do whatever they want. With my permission, of course.
Otherwise, I like it. When I was little, I used to lay next to my mother on the couch and she would idly run her fingers through my hair while she was watching the news, so it has a strong positive association for me.