I have no objection to posting. I learned to post long before I learned to sit the trot.
However, it is not a required skill for western riding. It is a good thing to learn, but I'm not sure how it would help a person canter better. I've met western riders who never post, and I rarely post for short distances.
Old time western riders used such a long stirrup that I would think posting would have been difficult. The saddle design would also have made it awkward, although not impossible.
"Pink Murray, famous wagon boss of the OR ranch in southern Arizona, mounted on his favorite horse...Arizona, 1909