I've always been taught to stand my feet about shoulder width apart except for driver. Wider stance, as far as I know, will give you more stability which gives you the ability to rip it a little harder on the tee box.
I always increase the width of my stance just a little with the Driver. This will giver you more balance for the harder swing. Definatley want your feet shoulder width with your mid to long irons, and then just as Bravo said keep feet close together with the short irons.