I Bend it a little: I keep a slight cup in my left wrist from address until the start down.
A key to consistency is controlling the clubface. You can do that by maintaining the angle of your left wrist from address until just into the downswing. I prefer this slightly cupped position so there's a bit of inward curve between my wrist and the back of my hand. It allows me to consistently hinge my wrists and set the club at the top as well as release the club through the ball. The hinging is crucial to creating speed.
I keep that cupped position for a foot or so into the downswing. Then I start to release the clubhead before my left arm gets down to horizontal, with my left side leading.
If your left wrist is in a bad position, it'll force you to make compensations on your downswing or risk hitting shots off line. A wrist that's too bowed can promote a shut clubface and hooks or pulls. If it's too cupped, you'll tend to leave the face open and slice your shots. Whatever wrist position you set at address, stay with it into the downswing.
__, a Golf Digest Playing Editor, is the golf professional emeritus at The Greenbrier, in White Sulphur Springs, W.Va.