I've found that with hinges, the battens need to push into the sail so that they are lying on the vertical line through the sail panels (if cut flat). This avoids having the hinges slacken on one tack and tighten on the other. So two grommets, one above and one below the batten, is better than one, I'd say.
But if you're having new sails made, why wouldn't you have pockets added - much more convenient and cost effective than lacings? In which case, I've found it best to make them deep vertically and to sew them flat onto the sail so that again, the sail is pushed out one way, the pocket in the other, and the hinges don't slacken and tighten.