In order to get the most out of your models, you should do all you can to ensure that they are comfortable in the setting. To prevent strains, muscle aches, and the moodiness that can result from physical discomfort, strive to pose them naturally.
Yeah, that's it, exactly.
(Actually, based on the little I've seen of America's Next Top Model, none of that is true at all. In fact, you should ask your models to contort themselves into bizarre poses in unusual settings. The less comfortable they are, the better. It builds character.)