WitherBoss ribcage (torso) + tail is approximately 2 pixels out of alignment with shoulders + heads, from center.
Discovered while creating new texture, initially assumed to be my own error, after a few hours of research and adjustments I have concluded that based on the texture resolution it is incapable of being the fault. Lack of WitherBoss model prevents end-user solution.
Hopefully this can be slipped into 1.8.2 due to the relative easy solution of simply sliding over the torso and tail. (As even without OCD this is driving me crazy! lol)
Attached is a screenshot of the most dead-on view I could obtain, and a line depicting center (between center-two pixels of the skull).