That's a good question. I don't care so much about shoulder tilt as much as I do head and spine tilt. Maddux was one who had some shoulder tilt but still kept his head fairly upright. I feel that as the head goes so goes the spine and probably the shoulders. Take care of your head and everything else will probably take care of itself. Head perpendicular to shoulders but everything tilted is not ok to me.
Another reason I like the head to be upright is that the force of gravity gives you a natural reference point that you can feel. You can probably stand up, close your eyes and find vertical more accurately and consistently than you could find a 20 degree tilt. What does that say about repeatability?