I thats an exact quote from your customer they're lazy. Thats lousy specification language. Creep under what conditions? If they really wish to maximize or optimize, creep properties then they should do their own homework not to mention provide you, their supplier, with the temp with which the material should be PWHT'd which is tied into their design.
However, this also ignores the balance that needs to be maintained between creep and toughness when considering PWHT. Current thinking is that the best balance is achieved at ~1400deg F.
If you want to "garantee maximum creep properties" then theoretically you don't heat treat it at all. That way all of your Cb, V, and N stays in solution. However, this would be insane. You have to consider toughness as well.