I have it where my face, jaw and head muscles all tense up too! What's with that? @martin I saw your PDF on heterophoria in Gurm's thread and it fits me pretty well.
The single worst contributing factor besides bad devices and lighting is wearing glasses! I can't wear any glasses with my astigmatism correction anymore without pain. Instantly I can feel all the muscles tighten up worse when I put them on, especially the extraocular ones. I have had that script adjusted by so many doctors and it makes no difference as long as the astigmatism portion is in there. Hyperopia correction is fine though for some reason.
I used to have a crazy acupuncturist who would needle the extraocular muscles by inserting the needle between the eye and the orbital bone all around my eye. The muscles would instantly release if needle hit the right spot. Got some crazy bruises that way, but it was one of the only things that ever worked. I think the extraocular muscles get tensed and actually move the eye (worsening heterophoria?) and they get "stuck" in the contracted position.
I have to tilt my head to the left to read as well. Compensation for heterophoria?