I think that it depends on what kind of quilts you make. Do you make lap-size quilts or full size? Do you make scrappy quilts or traditional patterns where you often need 5-6 yards of a focal fabric and varying amounts of other fabrics?
I tend to make full size traditional quilts and have bought anywhere between five yards and the whole bolt of a great focal or backing fabric. A great border fabric, at least four yards. Blenders or versatile small tomid-size prints, usually 2-3 yards. I never buy less than one yard. Unless I'm doing an applique quilt and only need a tiny bit of one fabric, I never buy fat quarters.