Your cruise C of G is related to the MAC (Mean Aerodynamic Chord) and essentially governs your max cruising level and hence your optimum cruising level. Recall the old 172 days with your lift/drag/thrust/weight couples. Well, when you enter a higher cruise C of G value (up to a point), it effectively shifts the lift vector closer to the weight vector. This in turn requires less downward force on the rudder and hence reduces the effective 'weight' of the aircraft (in reality the rudder produces less trim drag) and therefore you are able to cruise at a higher FL.