Actually, doctors who study X-rays (radiologists) are asking this same question. Most positions for X-rays have been determined by studying the bones of cadavers (human bodies preserved for study). With live humans, there may be a significant effect of the muscles acting on the bones.
Perhaps slight differences in position aren't that important. But until this question can be cleared up, it's necessary to follow standard procedures and keep the wrist "just so" during X-rays.