Have you ever encountered a situation where the "turn torso sideways" pose obscured an interesting piece of cosplay? It happened to me once, and I didn't have the heart to ask the cosplayer to adjust her stance.
That's happened to me, but I'd say you can always ask a costumer to adjust her pose. I love it when the photographer gives me some direction. When it comes down to it, I'm really just guessing at what she can see through the camera, so I'm always a bit relieved when someone says, "Look down, turn left, stand on your head."