I think that quite apart from any SOP or safety consideration it would be imprudent to close down a pack as a way of reducing fuel burn rates because of the effect on the paying passenger comfort. Used to use 1/2 pack flow on the Classic at times, however this led to a stuffy environment in the aircraft due to the reduced outflow, especially on high pax density loads. Shutting down a pack is not something I would use to save fuel, much rather cruise at best level/best speed for weight in the prevailing ambient conditions.
