All of the time.
Every chance to thank a driver should be taken. The more we try to make it less 'Us n Them' the better. I even nod at drivers for not pulling out in front of me when I have 'right of way'
Cheers - Lee
Also, if I'm at a set of Red lights and a cyclist blitzes through them, I make a gesture to any motorist in sight to show them that I freely acknowledge that it p1sses me off too.