As far as scooters, and any other motorized 2-wheeler; if its safe, and they are in a safe situation, I will wave. Who cares if they wave back. Being cordial must start somewhere. Now that being said, if there is a heap of bike traffic, I usually take the safe approach of keeping my hands on the controls. The occasional nod will have to do!
If I'm on a slow twisty 2-lane, I'll even wave/nod to the occasional car/truck; especially if they have the window down and an arm resting on the door. I waved at a guy in an older Porsche convertible last week (He ended up beside me at a light, and the smile of the old man, and the thumbs up, suggested that he may have been farmiliar with these 70's bikes). He changed lanes and followed me home through some nice twisties. He was a good driver, and we both had a good time, and waved again after the "carving" was through.
Fabricator