Icing the back before a workout would not be ideal and would probably lead to an injury. If it is due to an injury the question should be whether you should work out. If it is not due to injury, but instead tightness in the lower back then a proper warmup (movement prep) followed by movement drills (focusing on proper movement patterns) would be better. For example, when I suffer from lower back pain it sometimes feels like a pulled muscle on one side or a pinching feeling in the center of my lower back. While this is not always the answer to what ales you it does often help to do a dynamic stretch warmup for the muscles in the hamstrings. A great exercise for this is to stand straight up looking straight ahead. Bend forward at the waist while lifting one leg behind you. The hips need to stay square so it helps to turn that foot in so that you load the hip of the leg you are standing on. As you bend forward extend your arms out to the side with you thumbs pointed towards your back. Push the heel of the foot that is in the air out and continue to bend until you are standing on one leg, hips square to the ground, hip loaded, other leg straight and parallel to the ground, foot turned in, heel pushed out, torso level to the ground, arms straight out to the side with thumbs pointed up and head slightly up enough to see an object ahead to assist with balance. Don't hold it, but step back and swap legs. Continue to step back for 4-5 reps per leg. It should loosen up those muscles nicely, help to open your hips, and alleviate the back pain as well as getting blood to those muscles so you can conduct a safe workout. Save the ice for afterwards, and if you can do cold and hot contrast baths/showers. Ice after a workout will help to slow/stop the breakdown of muscle tissue. By contrast between hot and cold you are able to open the vessels and infuse your muscles with blood when in the warm then transition to cold to tighten (pressing toxins from the tissue) and also help reduce inflammation.