I say yes, it is important. If you are putting a lot of time and money into a quilt, you are going to want that quilt to last for generations. Quilts are usually heirlooms that are passed down to the generations. So yes, good quality thread is important as you don’t want your quilt falling apart after 15 years. You can test your thread buy wrapping it around your hands and pulling. If the thread breaks you probably don’t want to use it in your quilt. My favorite brand is Mettler and they make a nice cotton with a silk finish.