The position of the ball in your stance is an important part of playing your clubs correctly.  With woods off of the tee and in the fairway you need to place the ball at your inside left heel.  This ball position will allow your woods to get the proper ball flight and power.  Your irons are a different story.  Even though you can hit the ball with a low ball flight by placing it in the back of your stance or higher by placing the ball up in your stance, you want to place the ball right in the middle of your stance for regular iron shots.  With the ball right in the middle it allows the loft of the club determine the ball flight, with a three iron the ball will have a low trajectory and with the same ball position you can hit a nine iron with a high trajectory.  This will also help you determine how far you can hit each club properly.