"The higher the top of the chest--the highest point on the ribcage above the bench--the steeper the angle with which the pec/delt attaches to the humerus. The steeper the angle, the better, because of the increased mechanical efficiency of the contraction caused by the steeper angle of attack on the humerus. [A lever] exhibits greater efficiency the more closely the force is perpendicular to the resistance, no matter what exercise. So the higher the chest position above the arms, the better pull the pec/delt has on the arms.

The short version: keep your chest up when you bench."

--Mark Rippetoe, Starting Strength, 2nd edition