I'm not sure I would set out to 'buy a stash' as "Stash happens". In fact, if I had to start from scratch, I'm not sure I would know where to start.
In the beginning, I was making two charm quilts -- one with blues (ended up with about 225 different fabrics) and one with music theme fabric (at least 170 different fabrics). Since I only needed a 6" square from each I ended up with plenty of both buying 1 FQ at a time (i still have 2 drawers full of music fabric). Another time, I collected bright FQs during a shop hop to make a couple of baby quilts. Again, plenty of extra fabric left over -- ended up making 230 Flying Geese plus the piano key border for a twin size quilt, gave a stack of squares to a friend, another baby quilt and STILL had fabric left over, although not as much.
At one time I collected cat fabrics (FQs). I'm not positive what I was going to do with them. Still don't know, but I've at least cut into them. I also have a box of florals, which I'm not sure how I've accumulated. Others I've collected for samplers I'm working on -- 30's; red & white; black & white.
I've used a whole bolt of a gorgeous purple and pink batik over the past 10 years; Kona Cotton Snow has gone into a couple of 30s samplers. 15 yards of Kona Cotton black has also been used. But these are fabrics I've purchased with specific plans for. I'm not sure I'll buy the black again -- I do dislike the lint it collects. I keep thinking I have enough of the Snow, but seem to run out and I have no idea what I would do with more of the batik.