Don’t slump and hunch over! Oh dear, don’t overextend and puff out with a super arched back! If you just stand up straight, not too much and not too little, then your voice will work!
Not necessarily so. Posture is not a cure all. Yes, it helps greatly, but even those folks who can stand up straight do not always sing well. Good posture is like a face that is lifted. These are outward signs of healthy inner realities. And, we can use these outward signs like scaffolding as a building is being constructed. The building is most important, but the scaffolding can act like a guide, as do properly constructed and executed vocal exercises.
The whole point is for us to naturally stand up, a lifted posture as well as a lifted face that becomes the status quo indicating correct interior vocalism. Ultimately, we can stop fretting over posture and just allow it to be, with a minimum of conscious control. A beautiful response to free vocal function!