Ugghh!! I bite my nails. It is the worst habit. I try to break it and I just keep doing it, especially if I am nervous. And I play with my hair if I am nervous. That is hard to do now that I have it cut short, but I still find myself twirling my hair or trying to. I consider them bad habits because they are so obvious and it looks unprofessional when you do them. Plus it does not add to your credibility of being a cool cucumber under pressure if your nails are bitten all to heck and your hair is a mess from the knots in it.
These are nervous habits and as I work to become more secure I am sure they will diminish, but until then bitter tasting stuff and nail polish helps with the nail biting and maybe I just need to shave my head.