When can you tilt your shoulders, and when must they be level?
When the snow is predictable and the terrain mellow, you can relax by leaning your whole body into the turn. Total body inclination, which includes a tilting of your shoulders and head, is a fine way to edge your skis-as long as you're not leaning in so much that they slip out from under you. This is a sweet and relaxed way to cruise. But keep more pressure on the outside ski than you do the inside, so that the instant the snow changes or the slope steepens, you can assume a more upright stance.
The "reverse shoulder" technique, popular years ago, is still valid today. Turn your shoulders away from the direction you are headed. Where it is steep, reversing the shoulders keeps you facing down the fall line. Where it is slippery, this technique helps turn your lower body to control the shape of your skid. There is nothing old-fashioned about it: Moving your shoulders in what seems to be the opposite direction reinforces "countering" in the hip (December Instruction: From the Ground Up, Getting Centered), and lets you anticipate the upcoming turn.
When making advanced, carved turns at speed, keep your shoulders level. Level shoulders are a guiding principle. By keeping your shoulders on an even plane, you bring your spine into a more vertical position. This creates an angle between your lower body and your upper body, which helps with edging. Your head and eyes stay level too, keeping you balanced. Your outside leg should be long and strong, and your hips should slide to the inside.