Slouching has nothing to do with upper neck posture. There is an old song that starts: "The head bone is connected to the neck bone" An impact to the head also impacts the upper neck. Studies show that upper neck injuries contribute to PCS symptoms in up to 80% of cases.

Posture is about maintaining good head and neck posture so healing can happen. If you break a bone, they put a cast on it so it is stable as it heals. The upper neck needs that same stability so the ligaments and tendons can heal. Plus, poor head and neck position causes inflammation. One wants to avoid causing that inflammation at all costs.

For me, one of my problems was getting progressive lenses in my glasses. I started needing to tilt my head back and forth to find the focus spot in my glasses. This appeared to aggravate my upper neck until I got disciplined to support healing. I can hear the C-1 to C-0 joint grind as I move my head up and down.