I wave vehicles on, but with care. Only when I can see that it is safe, & I know they can't, or occasionally when I sense they're dithering because they're not sure of my reaction.
I wave people back at least as often. I've been overtaken in unsafe places many times, & I find that a signal when I hear them starting to rev sometimes works.
If I wave someone on & they don't respond in a timely fashion, I wave them back. "It was safe, now it isn't".
But mostly, I don't signal at all.
What really irritates me is when a car overtakes immediately after a passing place on a narrow lane. That happens a lot. Sometimes, I've seen a passing place ahead, looked over my shoulder, slowed down as I pull over into it - no reaction, run out of space, so I put my head & foot down, only to hear the engine revs go up. Doh!