No, that old wive's tale is easily disproved. The human body is not straight, it consists of curves. The main curve is between the buttocks and the shoulders. If the bed is too firm, then that hollow is not supported and the body weight is taken by the buttocks and the shoulders, leaving the unsupported spine to collapse. This results in lower back pain and is especially noticeable upon waking in the morning. A quality mattress will conform to your body shape, and keep your spine straight whether you sleep on your back or on your side. If firm was good then you would be able to (and enjoy!) sleeping on the floor.
