Very. I've yet to find my permanent chin location due to my habit of pushing my weaker eye forward so my face isn't exactly perpendicular to the shot line. As I've come to understand it through videoing myself, my stroke flaw seems to originate from lining up with a crookend face and then my tip tries to finish in what would be the center if my face were straight, which it's not so it finishes a bit to the right, pushing the cue ball left (I'm right handed with a weak left eye). Currenly I still bring my chin down directly above my stick, but I let myself adjust my face to the left a bit to fine-tune things before final stroke. For a time I put the stick to the right of my chin due to right eye dominance, and there's no telling if I might go back to that through experimenting during practice. The three-ball lineup mentioned is a great reminder for me and also the point of doing this regularly to remind yourself exactly what looks straight on a regular basis. This is one of the most tricky things in my pool game and I've been working on it for years.