i'll only bite them if one breaks, to even the shape out and make it less sharp, cos i don't carry a nail file around with me.
my brother didn't used to bite his nails but picked them instead, and because he was concentrating he always made this funny face and even if he was hiding his hands we knew he was picking his nails. every time my dad noticed, he'd make my brother put these gloves on (big puffy ones like boxing gloves), and though he'd cry, he soon stopped picking his nails because when he had the gloves on he couldn't play with his toys or anything so he learnt not to have to wear them in the first place.
at the time i thought my dad was really mean, but looking back on it it probably wasn't such a bad idea.