I believe that good posture is something that should be practiced by everyone, but especially by those who wear vintage. Prior to and throughout the mid-20th century, good posture was greatly encouraged, and many young women participated in manners classes, where good posture was a staple of the the coursework. A proper lady did not slouch. Since this was heavily unforced, both by society and undergarments (just think about corsets and girdles), fashion was built around proper posture, thus it is beneficial to yourself and your clothing to practice good posture.
This page comes from Ern and Bud Westmore’s 1947 book Beauty, Glamour and Personality.