H.I.G. Clearly states that hair must be restrained.
It's kind of a gray issue. Alot depends upon your personal inspector.
But for obvious reasons, we want to keep our hair out of the food.
Besides nothing is more embarressing than someone pointing out a hair in the food.
I have often wondered if it wasn't one of their own, or another party at the table. But having your hair restrained helps alleviate the question.