Without a good stance, you can't make a decent weight shift, and that will cost you both power and accuracy. The correct stance width is one that is wide enough to support your body throughout the swing, yet narrow enough to allow you to make a good pivot and weight shift. For as long as I can remember, people have been taught that the correct width is shoulder width apart, and that's not a bad rule. The problem is that your stance should change from shot to shot. If you're not sure how wide shoulder width is, err on the narrow side.
Most players use too wide of a stance and lose power.
My rule of thumb is that the longer the club, the wider the stance. It's also true that the longer the club, the more weight you need behind the ball at impact. For example, a driver is hit with a sweeping, level blow, while a short iron is hit with a crisply descending blow. To accomplish this, it stands to reason that your ball position must vary throughout the bag. When you address the ball just barely pass your hand between you hips and the butt end of the club.