Well, that would be good for hearies I guess, but I would just prefer to have it subtitled. Either of them are better than having her words repeated back through the perception of the hearing man.
I think the best thing would be to have the script written in such a way that the signing is not even subtitled or voiced over, and what the signer says is implied through actions and context and translators. This is the case with "The Heart is a Lonely Hunter" and "Mr. Holland's Opus." Such a medium is difficult to pull off, and it often relegates the signing character to a less verbal role in the movie. That can be used to advantage, depending on the context.