How many nods?
One nod up seems a little arrogant, too familiar, or disrespectful if done to a non-peer.
One nod down seems humble, but may just look like a lack of confidence, especially if you loose eye contact.
Several nods slowly may look like you know something about the person, especially if you are smiling, teeth showing, with eye contact.
But if you smirk and nod several times quickly, with an open eyed glazed over look, you might appear slightly insane.
No doubt about it, the nod is tricky business.