"When preparing the denim material from used jeans, make sure that you don't use the materials by the knees or the rear. If you're getting material from denim shirts, avoid the elbow areas. This material will normally be faded and worn and you will want to use stronger, more durable sections of the garments for your quilt."
more info here:
http://www.quiltedparadise.com/n/036-uid.jsp