The harder you push them into the sail ie. tighter string tension, the more -curve- (or Draft) you are pushing into the mainsail. If you want a flatter sail for very light or very strong winds, you don't want the battens too tight, just enough to pull the wrinkles out of the pocket. If you want more draft for medium wind, then you want them in tighter to push the draft into the sail, but sometimes when they are tight, if the wind drops, you may have to "snap" them over when you tack, by pulling back quickly on the boom, as they won't come over with just the light air pushing them.

It's a tough call from the internet, without knowing the wind and the stiffness of your battens.