Maybe this will help.
After all the problems with the air conditioning contributing to some recent accidents, Boeing changed a lot of procedures (Checking the press mode selector at the preflight...) This is one of the last aircraft where you donīt have an eicas that will warn you if something had been forgotten.
With switching the isolation valve to the open position you will end up in the same config in which you would have been with a normal engine start. (Since the NG can supply both packs on the ground, hence you need the valve in open) So if some start problems happen, the QRH can be applied right away. QRH is always assuming switches to be in the usual position. Technically it does not matter at all.
Hope that helps
repulo