Keep the angle: As Fred Couples shows, the position of your left wrist at address should be the same at the top of the backswing.
There's one common trait I've noticed in the swings of nearly all the great players, from Harry Vardon and Bobby Jones to Jack Nicklaus and Tiger Woods: The position of the left wrist at the top of the swing is consistent with its address position.
Fred Couples clearly exemplifies this principle. Fred's left-hand grip is noticeably cupped at address, and it's still cupped at the top, which allows him to release the club fully through impact without fear of hooking (flattening the left wrist closes the face).
Likewise, good players who start with a flat left wrist at address have a flat left wrist at the top. Starting with your left wrist cupped and flattening it going back, or starting with it flat and cupping it, is unnecessary manipulation, which can lead to inconsistency.
For a reliable swing, maintain your left-wrist angle.
FLICK, a longtime Golf Digest Teaching Professional and PGA Golf Professional Hall of Famer, worked with hundreds of amateurs and tour players including Jack Nicklaus.