Visible posteriority and stacking are both important. If [you] just draw lines and just look at disc spaces, you wouldn't catch it -- so VP and stacking are important.
Visible posteriority is often accompanied by other segments posterior - the resulting appearance is stair-stepping. The most inferior posterior is listed as posterior.
Case History is only as good as the patient remembers, so you can’t rely on that. A 26 year old spine may have a 20 year old lesion. You have to give the patient a spinal age. Therefore, you are going to have 60 year old spines in 20 year olds and 20 year old spines in 60 year olds.