Let me give a tiny little lesson on propriety. If you're a juror, don't aggressively nod -- either affirmatively or negatively -- during counsel's closing argument. And for God's sake, whatever else you do or don't do, don't start clapping during his or her closing. For so many reasons that I don't even feel like counting them.
But if you do, c'est la vie. It generally ain't gonna get you booted off the jury (!) or constitute reversible error.
However, it's still extremely bad form.