Actually, scientifically speaking, stretching is a highly controversial topic. Some scientist say you should stretch after workout's, some say you shouldn't. However, it is clear that it is not scientifically proven that stretching reduces injury or soreness. It is advised to be used intuitively, meaning that if it works for you, you should do it. Here in the off season, I train about 15 hours a week for triathlon, and I do not ever stretch, as I have never experienced any positive effects from it, rather I would sometimes feel "loose" in my joints and tendons the next day. However, with skiing, I do stretch sometimes, it really depends how I feel. It's hard to explain, but I just know when I have to stretch after a day of skiing.
I would recommend that you try what works for you, so if you had back pain after a season, then definitely try stretching and see if it helps.
I would never recommend stretching before skiing, it weakens your muscles, and the elasticity in your joints and tendons, so don't do it.