There are no absolutes in photography; there are only generalizations that can be used as guides to making better pictures. The Rule of Thirds, for example, is a generalization designed to get the center of interest or most prominent feature, such as the horizon in a scenery shot, out of the center of a photograph.
But the Rule of Thirds can be expanded to the Rule of Fourths, Sixths, Tenths, or Whatever. On a cloudless day, it may be appropriate to include only enough sky to provide a feeling of speciousness. On the other hand, interesting cloud formations can be enhanced by including just enough foreground to give a point of reference.
If a scenery shot is split in half by the horizon, it suggests that the sky and foreground are of equal importance. The result is usually a dull and somewhat confusing photograph because the viewer's attention is equally divided between two prominent areas as the same time. But it's possible to find instances in which splitting the image actually results in a stronger photograph.
The Rule of Thirds, or whatever, implies that there should always be some sky or some foreground in a scenery shot. Again, this is usually the case, but in some photographs, having any sky at all can create a distraction. This can happen when there's a strong pattern such as trees that make up a forest, or rocks arranged in an interesting formation.
Usually, depth perspective can be increased with the use of a wide-angle lens. This will only be the case, though, if there's a fairly close foreground object. A medium focal length lens can sometimes be used to bring a distant object closer so that it can act as a foreground object. A telephoto lens has a tendency to compress distance somewhat so that the feeling of depth will be diminished, but this is not necessarily bad if other factors in the scene go into the making of a strong photograph.
Time of day can have a great influence on the effectiveness of a scenery shot. On a sunny day, the best time is usually the three hours after sunrise or before sunset. During these periods of the day, sunlight is warmer than during the middle hours, and shadows can have a strong effect on depth perspective.
If the sky is overcast, shooting any time of day, morning, noon or night, would probably give you about the same effect. Shadows will be soft, and detail will be prominent in all parts of the scene. A bleak overcast sky can give a strong impression of the day, but if the clouds have some form to them, you'll probably get a more interesting effect.
And speaking of shadows on a sunny day, remember to try for forty-five degree lighting in distant scenery shots. This will increase depth perception in a photograph, and detail will show up in the scene.
Another rule has to do with the nature of the photographer: be patient. First of all, never hurry a scenery shot. Check out the sky conditions, and if the sun is obscured by a cloud, wait until it reappears. Examine the scene for the best vantage point from which to get a good foreground. When you find it, run through the zoom range of your lens and select the best focal length before shooting. Better yet, take shots at several focal lengths so you have a variety to choose from. Even if you think you've found the ideal shooting location, it never hurts to move around a little and try to improve your position.
What all this comes down to is another rule: What you see in the viewfinder is pretty much what you'll get on film. This should be your ultimate guide.