When you are taking photographs do you get into a rut of only shooting horizontally? I know at times I find myself doing just that. It is a very easy to do because it requires less effort or thought, especially if you are under time pressure.
How often should you take a vertical image? I say as often as you can. This gives you options later in post and if you use the image the vertical may work better for the cover of a report or book.
Vertical images themselves take on a completely different look to our eyes because we are so used to seeing thing horizontally. The next time you are out and about or working a shoot and time permits flip the camera up and take a few vertical!