This is what I do ... most of the time. I choose the fabrics and THEN I choose the quilt pattern to suit the fabrics. This is primarily because of 2 reasons: (1) both the time available and my mood to shop have to be just right, and (2) I buy, almost exclusively, quality fabric at sale prices. In this way, I have no alternative but to build a fabric stash.
While shopping, if both the fabric and the price are right, I cannot afford to pass it up. I then keep some snippets of my fabric stash in an envelope in my purse so, on the occasions that I do get some time to browse and shop for fabric, I have the samples with me to find matching fabric. On the rare occasion that I choose a pattern first and then go shopping for the specific fabrics for that pattern, it's usually difficult to find what I want with all the requirements: color, quality, and price (and I refuse to buy poor quality fabric ever again ... been there, done that!)
I quite enjoy my method of buying fabulous fabric on sale and then choosing what to use it for; in the long run, I believe it saves me money.