I think it would be case dependent, but as a general rule the foreground and sky should be visible (By Default) to see the sky for the point of view of clouds, sun position and its interaction with the terrain, and the terrain (Foreground included) for seeing surfacing, displacement and the like. I can envisage times where you might want to considerate on the terrain or the sky separately (To allow refinement of problem areas, or for certain production needs) the ability to focus on these areas (Via clipping planes or other methods) should given a properly designed renderer to free up system resources and considerate on those areas, weather this is in the 3d Preview or on the image at render-time, its about allowing flexibility of work-flow to meet the needs of the artist.
Regards to you.
Cyber-Angel