Different cultures put different direction as "up". If there's a large waste to the north of your culture and lots of nice areas to the south then it's perfectly reasonable to have south as "up". Or East might work better if your culture is relatively equatorial and oriented toward sunrise. Perhaps convention requires that all maps have a fixed location as top center (or at least up) on all maps.

After determining the correct world-based orientation for the top edge of the drawing surface then it's just a matter of making the artwork align with it. Highly-directional artwork such as isometric mountains are an artifact of the cartographer for the enjoyment of the user. Folks attempting to use such things for more than concepts and general overview will understand that things look odd when they turn it a different way in eaxactly the same manner that a painting of a horse looks odd when turned upside down.