It's nice when people do it, but I wouldn't trust someone flashing their lights at me to know when I have enough clearance. Years ago when I was driving through the Columbia River Gorge, I couldn't figure out why it seemed I was getting cut off by so many trucks. I usually travel with my lights on. Finally figured out that I had a short in my switch that would cause the lights to go off every time I hit a bump.
And honestly, as far as flashing the hazards for a thank you, I'd have to reach over to the middle of the console to hit the switch. In some situations, it's just not a safe thing to do.