I ride a lot of curvy roads here in the mountains. As cyclists, we are more keenly aware of what is ahead on the road so if I have a car behind me, I will wave them by as soon as I can tell that it is clear. I also give them a wave and thumbs up for their patience. I also wave and give a thumbs up if they give me plenty of room. A little consideration goes a long way.

Now, my son. He holds his hands out letting cars know not to pass or pull out when going through intersections. I've told him, that attitude will get people pissed off at him. At least IMHO.

Obviously if the road is crowded it is senseless to wave at everyone but I don't ride those roads too often.