In NYC, if I had to wave at every biker I see, I'd a) be riding one handed for most of the way and b) have died months ago dragged under the wheels of a truck as I waved and lost control.
When I'm on my roadie and I see another person riding out on his roadie, I'll usually wave, but I'm not gonna wave to the 20000 immigrants delivering fast food every day, there's only one of those I usually wave at and it's the guy who delivers fast food near the place I usually have sushi at that I helped fix his brake last winter when it wasn't working and it was snowing.