Yep, we have been known to do it. and we are thankful when someone flashes us, regardless of the speed we are doing (which is normally the speed limit).
If we do flash its for the commuters, families and definately the truckies. And mostly we will only flash when we are in regional areas where it is more a common courtesy. And we flash for all safety things. I like being flashed when there is a safety issue up ahead as well. And yep I definately give the truckies a thank you when I overtake them, followed them in hope they would be my animal bouncer, etc.