Is the subject the sky above the land, or the land below the sky? If you have an interesting sky that you're trying to photograph, leave a third of the photo for the ground. If you are shooting terrain, leave a third of the photo for the sky.
Again, that's a general guideline. There might be other things that change how you frame the shot—a large aircraft in flight might cause you to leave a little more ground out of the shot. A strange cloud pattern in the orange light of dusk with a flock of geese crossing it might do best with no ground in shot at all!