Tail-heavy is nice for fuel burn, and should be what all load planners strive for, but other operational requirements of ground handling may (depending on carrier) carry more weight than chasing an optimum ZFW CG. This will be particularly true on shorter flights, where the fuel burn benefits of an aft CG are not as prominent as on a long-haul flight.
Some operational requirements that may run contrary to optimum CG are: Priority baggage loading (F, J and short connection bags loaded in the door position), separating mail, freight and bags in different holds, late connecting freight, mail or bags, carrying return catering or fly away kits. etc.
Of course, if you're flying LCC or charter it's relatively easy: Fill up the rear and stuff the rest in the front.
As mentioned above, the box needs to know ZFW CG so it can add the fuel quantity at any given time to obtain actual CG and thus trim setting.