Swedish Steve:
But it isn't. The B744 stab tank starts to empty in the climb, as soon as there is room in the centre tank. The reason is that if the stab tank does not empty for any reason, the aircraft will go out of CG limits later.
Fair enough, but I'm sure this is done for reasons of practical simplicity. The most fuel-efficient method would be to drive the CG to near the aft limit, then maintain it there as long as possible via split feed.
I recognize this may not provide required contingency operations, though; and the practical difference in specific range may not be significant.