My understanding of tail (trim tank) fuel is to reduce the amount of trim and therefore drag, not to swap pitch up trim for pitch down trim. The a/c still needs to be stable. Especially air transport category, to comply with regulations. Think about failure modes. If engines are lost with the a/c in non normal trim, hand flying, whether it be fly by wire in direct law or traditional control systems, will be very tough for an average pilot. Military a/c performance is a completely different kettle of fish.