I think it would come down to when he "dipped his shoulder". If in the umpires judgment the dip is after he has already step toward first (thus committing himself to throw to 1st) and then completes the throw, there is no balk. I guess if judged that the dip came before the commit to 1st then you can call a balk but the way you explain it I wouldn't. LHP have an extreme advantage holding runners at first, that is just the way it is. Some would say Andy Petite balked on every pitch. Check the box score... he didn't.