As a bike commuter from UW Bothell to Sammamish I ride the Sammamish River Trail home at night. There are many trail riders coming north in the opposite directions. About 1/3 of the riders will cover their headlights to avoid blinding me, but 2/3 of them are blissfully ignorant or don't care.
I always cover my light. I shout at the really bad ones. Up to yesterday it was merely an annoyance, but yesterday I almost ran down two pedestrians going in my direction because I had covered my light and simulateneously was blinded by an oncoming cyclist as I approached the walkers from behind. I only saw the pedestrians at the last minute and was able to pull off onto the grass.
Please cover your headlights when you see oncoming cyclists!! Even relatively weak headlights can be blinding when they're pointed in your face.