I think one of the oddest responses to a verbal greeting is (and this is almost exclusive to males,) the sudden backward jerk of the recipient's head as though he had been physically stricken by the greeting. I suppose this is a modification of the affirmative lifting of the head to signify much the same thing. I just think it is odd.

Having lived in a broad variety of social environments, I would have to say that responses must be geared to the local norm. If a casual nod or (index) finger-flick is generally accepted; do it. If local folks attempt to recreate a scene from the writings of Joel Chandler Harris unless you say something aloud; speak.