Well, they use 1.2Vs and not 1.3 for a start! And don't forget the heavier you are the faster the FMC tells you to go. Just from memory, subject to no forecast bad weather/turbulence etc. we would often climb when the FMC showed the max. alt. to be 1000' above what we were wanting, I stress, NO bad wx! From a fuel consumption point of view, with the PW engines, from my experience, it is less of a penalty to be high early and fly into optimum than to be trapped at a lower level and getting lighter all the time, watching optimum alt. spiral upwards and away from you, with all higher levels occupied by the remainder of the South East Asian nightly armada! (Never had a problem with levels SIN/SYD/MEL personally, non-standard often available, room for everyone).