It should be as close to flat as possible. Too great an angle, either from the keyboard itself or from misadjusted seat and keyboard height, coupled with years of the repetitive motions of keyboarding, can result in inflamed wrist tendons. This is the first step in developing carpal tunnel syndrome, a serious and painful wrist disorder. Some keyboards have jacks or lifts that tilt the keyboard to an unnecessarily steep angle, and put undue pressure on the wrist. Be wary of these.
Many people use padded wrist rests for the keyboard.
However, keep the hands floating above the keyboard when typing. Do not attempt to type with wrists on the wrist pad. In light of these guidelines, analyze your current typing style. Your chair may be too high. Perhaps your desk is too high for you, or you are too close to the monitor with the chair pushed in.