I almost always buy the fabric because I love it and then figure out what to do with it. I can only think of once in the last year off the top of my head that I bought yardage specifically for a particular pattern.
I saw a fabric just today that I love and I was shopping the fabric as I was designing it in my mind. I kept changing the yardage of different fabrics if I changed the design in my mind. I finally thought... I'm going to stop and not order the fabric until I can get home and do some work in EQ6 to at least get a few design ideas where I can see them first! Then I'll get that fabric. But still, the fabric came first and then the ideas to use it.