Agree with all of the above - probably will have to remember it myself as I am anticipating a similar issue later this year with one of our SNCOs.
However, I must correct one misconception. It is not your sgt's appraisal, it is your appraisal (or whoever wrote it) of them. Whether they like it or not, it is not for them to accept or refuse it. If it is an honest appraisal, it will stand on its own when it reaches further up the chain. Plus your sgt is not signing to say that he agrees with the report, merely that he has seen it and has been de-briefed on it, a common straw to clutch at when the subject feels agrieved.
Either way, I don't envy you!