I believe that the relationship of the shaft at 1st parallel is determined by how a player "times" the "in" vs the "up". Couples has a lot of "up" early but no "in" until very late - hence this shaft is pointing "left" at 1st parallel.
Snead on the other hand (along with many great players) has early hip turn creating a lot of "in" before the "up" - hence the shaft points "right" at 1st parallel.
Only a "machine" with perfectly timed "up" and "in" would get it "on target". Many players get it there (or close) by manipulating a little during the takeaway on the assumption that this is key to being there on the downstroke.
Although "staying on plane" during the early backswing is not to be discounted as totally meaningless, the most important thing is to arrive at a position at the top (end) where one can move on plane in the downswing.
Do whats necessary to get the to best top (end) postion for you and do not get hung up on "position" golf in the backswing (been there - done that).
Its like worrying whether the clubface should be "open", "square" or "shut" at the top - whatever works.
Bruce