Sometimes I do, but only if they get chipped or cracked and I'm trying to remove the part that's broken. It's mostly because it can be gentler than just pulling at it with another finger, and I don't carry anything like a nail file or a pair of nail clippers. I don't really do it out of habit so much as out of fear of getting a broken nail snagged on something.
In other words, I use my teeth to cut/file my nails due to lack of proper tools, and that's the only time I bite them. Incidentally, I've also been known to use a BIC mechanical pencil with the lead removed in order to clean out the gunk from under my fingernails between classes. It's the perfect size and shape for the job.