A. Freud (who, by the way, was rarely without a cigar in his mouth) would say that persistent mouthing of objects beyond babyhood indicates an oral fixation stemming from arrested sexual development. But even he himself admitted that this was only a theory. So, sometimes hair chewing is just hair chewing. Your daughter's mouthing is probably a nervous habit that will abate in time, but it could take a while.
Our oldest, Madeline, who's 10, was a mouther until last year. Now, instead of putting objects in her mouth, she does this weird rolling thing with her tongue, which is annoying but somewhat more discreet and less germy than chewing on pencils. Since beginning piano lessons, she has another new habit -- "playing" piano on her legs, on the dinner table, wherever her hands happen to be. But we approve of this one.
As habits go, it all seems pretty benign to me now. You could try to get your daughter to trade hair chewing for, say, hair twirling, but it probably just has to run its course. We all have our personal tics, and children are entitled to a few of their own.
Even so, you might take note of when she tends to put things in her mouth -- at night, to soothe herself to sleep, or on the way to school, when perhaps she's worrying about something. If she seems increasingly anxious or emotionally withdrawn, consider checking in with your pediatrician or seeing a school counselor to try to figure out what's making her nervous.
In the meantime, show her other methods of relaxation. Madeline, for instance, is a big fan of hand massages with lavender-scented lotion. (And once I rub the lotion on, her fingers taste much less appetizing.)