I am not a professional golfer. The instructions contained here are no replacement for lessons. If you're serious about the game, there is no substitute for lessons from a qualified PGA professional. Talk to the pro at your local golf course or country club. No animals were killed or maimed in the making of this website.
Many instructors say a cupped left wrist is fine.
What's important to note is that while the back of these men's left wrists may or may not be perfectly flat, they are flat-TER at the top of their swings than they were at address. This is particularly evident in the following sequence of photos of the late Payne Stewart, who possessed one of the classic swings of all time. Here, you see Payne's address position head-on. Then, at impact, his wrist is completely flat, or even slightly bowed, as he's leading through impact. Below, you see his left wrist position at impact.