I think generally the more an individual cants the bow the more aware of head position you have to be. Think about it. As you go further towards horizontal, if the anchor stays the same, your hand doesn't stay in direct alignment with the eye. I think thats where most people notice a change in impact.
When I shoot at more and more severe cants I slightly change my anchor up on my face so it keeps the arrow under my eye. Easier to do than explain.
Chris