I sometimes also feel that emphasizing the feelings of posture too much might mislead some people into concluding that Zazen is about attaining some particular physical sensation of balance of body. I think Uchiyama makes it clear that it is really not about that, even though he was a "return to the posture, again and again" fellow.
I want to mention that these days I am emphasizing somewhat more that folks new to Zazen should follow the breath, and stay with that for weeks or months, gradually transitioning into "choiceless, open spacious awareness". In our video series for "Beginners, We are All Always Beginners", I tell people new to Zazen to follow the breath at the tip of the nose, as it enters and exits, but do not say audibly inside your head "inhaling" or "exhaling" or the like, and just be silently aware. Also, have some subtle feeling in heart that "inside" and "outside" are not two, and that the border we create between is rather an arbitrary construct. Just breathe, feeling that "inside is outside, outside just in".
Perhaps, once or twice a sitting, move off into "open spacious awareness" in which the "object of attention" is any one thing or everything in your surroundings, but without giving particular thought or pondering particularly any of it. Here too, maybe have some sense of "inside" and "outside" as not two. Move back to the breath again if difficult to rest there.
Gradually, folks new to Shikantaza should transition to "open spacious awareness," but it is fine to move back to breath on days when the head becomes particularly stormy and filled with runaway thoughts.
Gassho, Jundo
SatTodayLAH