Lisa Marie,
To answer your question, I buy fabric depending on the intended use of the project I am making. For large or time-consuming projects, or things I care a lot about, I always buy fabric that is nice and thick, with a high thread count and thick threads. This usually means name-brand fabric, like Moda. You can usually tell by a combination of feel as well as how well you can (or can't) see through the fabric. For wall-hangings or quick and easy projects, I gladly buy lesser quality, cheaper fabric from CT, Joann's, or other discount stores. Even when buying LQS quality fabric, I sometimes deal-search using websites like fabric.com, and I have found some fabulous deals on name-brand fabric at fat quarter shop's sale pages. The way I look at it, if I'm making a quilt I want to last a lifetime, I need to invest in the best quality materials for the project. Cheaply made fabric will hold up for a while, but eventually is likely to fade, wear thin, and fall apart.
For thread, I only use Mettler and other threads that produce just as little lint in my Bernina, because that's what my dealer told me to do. I view my Bernina as an investment that I need to take care of, so I do what I'm told. In my other machines, I am willing to use lesser-quality thread, but only if I'm using lesser quality fabric. If I'm using high-quality fabric, I use high-quality thread. I tend to try not to mix qualities in my materials in any given project.
HTH!