Around here, on the road, I can't recall anybody ever waving. On the several less-populated MUPs we have, I usually nod at everybody coming in the opposite direction -- maybe half nod back. On the few heavily-populated MUPs we have, there is never waving or nodding because of so many people.

That said, lately, I've been riding my Surly Big Dummy, and I've had several Serious Roadies In Full Kit pull up and want to chat about it as we ride. They've been genuinely curious and I've had some nice conversations.

If somebody wants to interact in a pleasant way, all the better. If they don't want to interact, no worries.