Its obviously best to lean forward, but there are definitely times that you are better off leaning back. If you are hucking cliffs and landing in multiple inches of sugar, you need to lean back or your skis will literally stop moving when you hit the ground. If you are skiing super chopped up snow at highway speeds, you should at least have your weight centered, but you may have to even lean back if you are on super short soft skis. And like everyone else said, if you are skiing through deep snow slowly, then unless you are on a ski that just won't sink you need to lean back. There are times to lean back, you just have to know what your doing. People who think its never okay to lean back are a bit naive.