Woodpecker,
An "Aft C of G" doesn't mean the tail has to produce force in the up direction for straight and level flight, unless you're loaded well outside allowable limits.
Assuming the aft C of G position is within the W and B envelope of the airliner, it just means that the tail is required to produce less down force than for a forward C of G. Less down force = more fuel savings, among other things. The use of the term "aft" just means the C of G is towards the rear of the allowable range on the MAC. Similarly, "forward" means the C of G is towards the front of the allowable range on the MAC.
We're not talking F16s, canards etc