I have heard these words from choir directors, vocal coaches, and other well-meaning folks during my career, and I always cringe when they are spoken. This was my thinking about two years ago. My voice started to let me down. Good posture is crucial because without it, the singer cannot breathe or support efficiently. Slouching or sitting causes tension, which adversely affects the function of the voice.
Some people say that singing is all about the breath.
Breath support comes from the diaphragm which is located under the front part of the rib cage. When I first began voice lessons, I didn't know how to breathe using my diaphragm. My first voice teacher taught me by having me pant like a dog. She explained that is what my stomach would feel like when I am breathing using my diaphragm. I used to rely on the muscles in my larynx alone to supply the power to my voice.