In fact, the pelvic area should be tilled slightly forward. The internal organs will be properly aligned in the vertical plane. This will ensure that your stomach area is not compressed, which would prevent proper breathing. Finally, there will be the problems associated with individual issues of where and how we hold our tension. Doing zazen can put us in touch with things about our postures that we never noticed before.
Most people hold one shoulder higher than the other.
In a person who centers tension in the stomach, there is a tendency for the pelvic area to slump, so that the Spine curves outward, when viewed from the side. People with hip problems tend to sit on one hip, thereby negating the principle of a tripod. And everyone has problems with flexibility in the knees and ankles. Be aware of the problem each time you sit and try to correct it. But don't become obsessed with it.