When shooting with this lens (20mm f/2.8), I always concentrate on the foreground because details in the background would usually be too small to see really clearly anyway. When I did this kind of scene with a view camera on film, I could adjust the controls (swings & tilts) to create infinite focus with almost any lens. This cannot be done with a digital camera, so, since objects that are closer to the camera appear larger, it’s better to focus on them. When working with a wide angle lens, you usually want to have the foreground as sharp as possible. Sometimes the foreground can be rendered soft on purpose (such as when you want to concentrate on a subject in the middle of the scene). In that case, the foreground is not as important as the main subject and the soft focus can force the eye to concentrate on it.
In this image, the rocks in the foreground serve to help frame the image and because they are closest to the camera, they actually become the main subject.
Wide angle lenses make you think more about what you want to show in a scene. Keep in mind the items above and do some experimenting to see what kinds of results you can achieve.