If you are picky about getting the best viewing experience, eye level at the middle point to 1/3 from the bottom of the image is best. It gives the greatest feeling of dimensionality and realism.
Think of it like a classical painting. The lines that give all the depth cues have a "vanishing point" (the line of the horizon which all the lines travel away from you toward) that are painted from the perspective of the painter/viewer standing directly in front of the painting. Of course the painting is a 2D image, but from the right perspective the vanishing points are correct for the viewer perspective and help give that illusion of depth. Look at the painting off angle and the vanishing points/depth cues are still visible, but they are no longer in the proper perspective to the viewer and the depth effect lessens.
Same with the images on any display, from my experience. You can "see" all the depth cues from most angles, but they aren't quite correct unless you are right in front of the image (as a camera-man would film it, seeing what his eye sees through the lens). I've always been amazed how if you adjust a display, or yourself, to be right in front of the display at the right height, how the dimensionality suddenly "locks in" in it's correctness and believability.
Again, that's if you are picky about these things. I am, which is why I am designing everything in my new HT room with the goal of having the screen not too high, but in the right relation to the viewer to maximize depth of image.