We're in early stages of treating visual processing issues. Many of the flags on the screening quiz could relate to the eye-rubbing you describe. Aha - I managed to find the same checklist we were given on a random website; I think this is fairly evidence-based. See if any of this looks familiar: http://www.centerforunlimitedvision.com/30question.pdf

You can also check a few basics simply by moving a pen back and forth and figure eights between the eyes, and in and out from the nose, and see if the eyes are working together. (Our DD flunked this 30 second screen entirely, shortly after a full check-up from her regular optometrist).