I was teached and prefer to select the final cruise altitude straight away, so FL400 in your example. Some captains I fly with, prefer to select the initial cruise altitude. When I ask why, they say that it is to prevent an 'OFF SCHED DESCENT' annunciation when you don't make it to the final filed level.
However when you do it with reselecting the flight alt and you step up from at or below FL370, climbing to FL380 or higher, you will get an annoying pressure 'bump' because the pressurization controller will have to reschedule a new differential pressure (from 7.8psi to 8.35psi) which causes the cabin altitude to change in an uncomfortable way, because it usually forces a cabin descent to increase the cabin pressure and thereby increasing the differential pressure...
When you step up from, for example, an initial FL380 to a final FL400, it will not give the annoying pressure bump, because then the differential pressure will remain the same
I think the reason of avoiding the 'off sched descent' is a bit too cautious... Usually you will always climb to the final planned cruise altitude. However if you do not, which will be a deliberate decision, you have plenty of time to reset the flight altitude window. And even if you forget to do that, the annunciation coming on doesn't harm anything, just reselect a flight altitude just below your actual level and the amber light will go out and the controller will regulate a smooth descent to your planned landing elevation.
Last edited by reivilo; 7th September 2010 at 20:47.