On QF if we have to do a manual trim sheet it is done by the pax zone breakup, if it is computer generated the trim is done by rows. The people on check in normally seat everone fwd as everyone wants a fwd aisle seat. I haven't heard of lateral imbalance due to the fuel load or part of the fuel system U/S but I would imagine if the system did have a problem engineering would be required to fix it. I do know that on the 767 if ld3 (ake type) cans are used then there is an adjustment to the mtow by subtracting 150kg for each ake used, due to lateral imbalance.