Always on. 100% of the time, no exceptions. Screen tearing and motion judder are the greatest evils in the history of mankind.
People who play with Vsync off don't deserve PC gaming! They clearly have no standards regarding IQ and performance. The input lag is unfortunate, but nothing is worse than screen tearing and the inconsistent judder you get from fluctuating frame rate. That shit is the destroyer of worlds.
I really wish that there was a G-Sync television on the horizon though. I'd love to be able to turn off Vsync and not have to deal with tearing or judder. Maybe some day.