1. Is it best to match cotton thread to the cotton fabric?
2. Will polyester thread really tear the fabric in my quilt?
3. Is shrinkage a factor to consider when selecting thread?
The number one reason why quilters are taught to use only cotton thread is due to tradition. Remember the story of little Johnny watching his mother cut off the ends the roast before putting it in the oven? He asks his mother why she does that and she replies, "Because my mother taught me that it tastes better this way." Johnny asks Grandma the same question and gets the same answer. Johnny then asks Great Grandma and she replies, "Because that's the only way it would fit in the pan." Sometimes tradition is stronger than reason.
If we trace the history of quilting back to great grandma, we learn that she used only cotton thread. Great Grandma taught Grandma who taught Mom who taught me and the lessons and traditions follow through the generations. There is a reason why Great Grandma used only cotton thread. That's all there was at that time. If we use cotton thread because of tradition, that's not a good reason. If we use cotton thread because we like it, that's a great reason.
What about shrinkage? Quality fabrics do not shrink like the old days. Most are pre-treated for shrinkage. And even if there is some shrinkage, it isn't realistic to expect a woven cotton fabric and a twisted cotton thread to shrink in equal proportions. Use quality fabric and thread and there should be minimal worry about shrinkage.
Will polyester thread really tear the fabric in my quilt?
Thread will not tear through a fabric solely due to its fiber content. If a thread ever tears through a fabric, it is because it won the strength contest. In a battle of heavy use and high stress placed on a quilt, the strongest component will always win. It is not accurate to say that a polyester thread is stronger than cotton. I have a spool of thread on my desk labeled 100% cotton quilting thread. It is a very well known brand, widely available, and is one of the top selling machine quilting and piecing threads. It is twice as strong as a comparable polyester thread. Although it isn't labeled as such, it is coated with a glaze which strengthens the cotton fibers and makes the thread rather wiry. In a strength test, it beats a comparable size poly, rayon, and metallic every time. In a heavily used quilt, this stiff, wiry cotton thread could do more damage than a soft polyester thread. And it is 100% cotton.
The point is this: The traditions, myths, and rumors that polyester thread will tear the quilt are not true. Under extreme use, a strong polyester thread might, but so will a strong, glazed cotton thread. Under normal use, softer non-glazed threads will not tear through the fabric and it is perfectly fine to use any type of quality thread, except nylon. Nylon will go brittle and discolor over time. Choose your threads based on quality, feel, and appearance, avoiding wiry glazed threads. Although they are rarely labeled as such, if the thread is stiff and wiry, it most likely has a glaze coating.
Whether you are creating a showpiece or a daily-use quilt that will be put to the ultimate tests, it is perfectly fine to use quality polyester or non-glazed cotton thread. Which fiber type will last longer? Poly will last a lot longer than cotton. But then, after 80 or 100 years, the quilt has served us well and we should expect some deterioration. What good is a masterpiece that is locked away in an air-tight closet? Quilts should be enjoyed and shared. That's how memories are made.