This guideline would be difficult to exceed in a home theater, with most vertical viewing angles in the 15-20 degree range. See the diagram below for an example.
The THX cinema standards have no references to a "best" or "optimal" vertical viewing angle. However, normal human vision's optimal cone of vision has a radius of approximately 15 degrees. From this I'd suggest that anything in the range of 10-20 degrees would be good. With larger values causing increased eye and neck strain.
Side view of a typical front projection home theater. The same concepts hold for rear projection and flat-panel displays. A diagram such as this, drawn to scale, is a good way to compute the minimum screen mounting height and the vertical viewing angle.