Here is a good read for you naysayers...
Myth # 4. Wide grip works the outer pec and narrow grip the inner pec.
The typical belief is that the wider the hands the more you will hit the outer pec and the narrower grip will hit the inner pec. Well there is a major problem here. The Pectoralis Major has two portions a lower, the sternocostal head and an upper, the clavicular head. There are no inner and outer pecs anatomically speaking. Let me define a narrow and wide grip. In two separate studies, researchers determined narrow grip as the distance between your acromion processes (slide you hand down your trap and the bony bump you hit is the acromion). They then applied this measurement to the hand spacing (distance between index fingers) on the bar. Wide grip was two times the narrow grip distance. Both groups of researchers found that grips that were 1.65 to 2 times their narrow grip were the most effective strength wise. The way you can determine your grip is to measure the distance between your acromion processes. Now measure the distance between your index fingers when you bench. Divide the bench distance by the acromion distance and if your number is between 1.65 and 2.00 you are in an optimal position (Clemons, J. & Aaron, C, 1997; Wagner, et. al, 1992). In the research, the activation of the upper and lower portions of the pec muscle are affected by different hand spacing. In one study it was found that the wider grip placed more stress on the sternocostal head than the narrow grip. The narrow grip seemed to activate the clavicular head more effectively then a wide grip. The narrow grip also activated the triceps more than the wide (Barnett, Kippers & Turner, 1995).