Using the DOF Preview button to make compositional choices

You've all seen images like the pair below where one part of the image (the foreground or background) is in focus and the other (background or foreground) is out of focus.
You as the photographer have the choice of where you want to direct your viewer. In one instance, the foreground is the subject and the background offers context. In the other instance, the background is the subject and the foreground offers a point of view. It's all up to you. But how out of focus do you need or want the background or foreground to be?
This is where the depth of field preview button comes in to play. Choose your subject, either the foreground or background, and focus on it. The other object should be out of focus. If it isn't, then you're not close enough to your foreground object.
Now pick an aperture. It doesn't really matter what it is (unless it's wide open, because when you use your widest aperture, what you see in the view finder is what you get in the final image). Press your DOF preview button and see what your chosen aperture does to your intended out of focus area. Is it too in focus or is it so out of focus you can't tell what's there? Change your aperture and check the DOF with the preview until you get the effect you desire.

Nikon F5, 80-200mm f/2.8, Fuji Velvia