A little is effective but make sure it is only once in the book. You would be surprised how readers pick up on repeated phrases and tire of them fast. Even habitual nail-biters can be darn annoying.
Here's a handy chart that employment agencies use when people are going for jobs.
Open palm: Sincerity, openness, innocence
Tapping or drumming fingers: Impatience
Patting/fondling hair: Lack of self-confidence; insecurity
Stroking chin: Trying to make a decision
Looking down: face turned away Disbelief
Biting nails: Insecurity, nervousness
Pulling or tugging at ear: Indecision
Adjusting tie: Insecurity, nervousness
Putting tips of fingers of one hand against tips of the other hand: Confidence
Clearing throat: Nervousness