What is doctrine on ordinary daily posture? In this question I'm asking about posture -- holding the body straight, for example -- not especially asking about posture during formal meditation, which was the topic of previous questions.



I was surprised to read this answer, which includes,


  ... walking straight, aware of body posture, like a king/queen, straight, not like a cowboy or a pig, awareness of the body and correct it.


Sometimes (though, not often) I see something similar in other answers, for example:


  You need to develop a good posture, good grace, and good gait. You should sit, stand, and walk conveniently, with good energy flow.


I was surprised to see that because:


I don't remember that's being a doctrine in the suttas -- at most maybe I remember comments on (or descriptions of) the Buddha's or Rahula's physical appearance, e.g. "like a lion" etc.
Buddhist doctrine emphasises view, and knowledge, and wisdom and so on -- if there is a mind-versus-body dichotomy, I'd expect Buddhism is more on the "mind" side of things.
Some doctrines seem intended to deemphasise the value of the body -- meditations on the foulness of the body, contemplation of corpses, etc.




So -- what is doctrine about posture (from suttas, from what you've been taught, and/or from your personal experience)? And, do you know, why? And what is difficult about that practice? And does that affect your social relationships, somehow? How does it interact with practicing prostration?
In the Theravadan tradition, most of the rules governing posture don't come from the suttas, but rather the vinaya.  Specifically, they are found in the 75 sekhiyas.  The first 26 are almost all about posture.  

The reason for these practices is really no different than those related to appropriate posture during seated meditation.  And it has a similar set of challenges.  In the West, we often set up a dichotomy between the mind and body.  No such division exists within Buddhism.  Form is just another skandha.  Just because Buddhism meditates on the foulness of the body does not mean that it should be ignored or viewed as somehow less important.  The body is also a tool that can be used on the way to liberation.  A posture maintained ardently, with mindfulness, and clear comprehension will have a comparable impact on cognition.  It establishes alertness and vigor while a bodily posture that is not meticulously maintained falls into sloth, heedlessness, and dissipation.  An upright body is an upright mind.  They are not different things.