My MO changes from one project to the next!! If it is a wallhanging, then I usually have/buy a pattern and pick fabrics from my stash to make it. Once I get to the borders, I am off to the store with my top, looking for something to finish it off.
If I am making a throw or bed size quilt it is usually because I have found a line of fabric that I like. So, I buy a couple of jelly rolls, a fat quarter stack or layer cakes, some fabric for borders, etc. THEN, I go home and look through my patterns, books and magazines for inspiration. Usually I get an idea then I am graphing on graph paper to make it the size, shape, etc that will work with the fabrics I already purchased (is that bass-ackwards or what?!?!?)
I do not think I have ever purchased a pattern, found "just the right fabrics" for it and then took it home and made it by the pattern!! That would be too "normal" for me!!!!!