I come up with my design first too and then buy fabric for it. I also usually overbuy, so I end up with a bit more in my small stash. Besides the fabrics that are earmarked for a specific quilt, I don't really have enough of any one thing in my stash to make a quilt with.
However, I LOVE scrappy quilts, and my LQS has a huge bin of scraps that I go diving in every couple months. This way I get way cool fabrics for my scrappy quilts without paying for whole pieces of them. I would say i have a pretty unreasonable amount of scraps, especially considering most of them are not MY scraps!! LOL I NEVER turn down an offer for good cotton scraps!! I use everything that is one inch wide or bigger, and I have gotten some pretty awesome fabrics by collecting other people's scraps. If they had a store, that was like a candy store, where you could go in and fill a bag with scraps for a certain price, I would be in big time trouble ....
Cheers!
Rachel