I just talked it over with the lead instructor at my AM. He said that although the spine has natural curves, the dancer must try to minimize the curvature of the spine for the correct dance posture. But a straight spine used for dance posture is not a posture you will carry all day long in your everyday activities. He said for one thing we would look funny. He agreed that there is a certain curve that exists naturally in the spine and a person would not want to try to "minimize" it all day long. So, the statement "Try standing against a door, everything touches. Walk away keeping that shape. Does it feel strange. Thats how it should be even when you are out shopping" applies for vertical alignment of all body components, but does not mean trying to straighten out the spine all day long. In a nutshell, good posture does not mean a straight spine, it means good stacking of all your body components vertically while maintaining properly curvature where its supposed to exist, in the lumbar and cervical regions.