I would think that anyone that uses a "string peep" would have to tilt his or her head inward. If you hold your head straight and upright, proper eye to sight alignment most likely will not be correct.
Thats true if your peep and sight are not setup correctly.
When I setup my peep/sight window I do so by drawing my bow back with my eyes closed and my head straight. When I settle into my anchor I open my eyes and if everything doesn't line up I adjust the peep and/or sight (pinguard) until it's perfect. I do see alot of people who adjust their head instead of their equipment.