I started really building my stash at the point that many Walmart stores were deleting fabric from their inventory. My local store fortunately was not one of those and got lots of fabric from the close outs. It was selling for from $1 - $3 per yard so I stocked up big time. In my inexperience with quilting I did purchase some that was less desirable but it has still been used in scrappy quilts without major problems. Did need to do wider seam on some that raveled easily. And discovered that one melted with too hot an iron. Oops!
But the one thing I learned was that I tend to be drawn to "focus fabrics" and have to remind myself to shop for blenders. I don't use many solids because I prefer the softer, more textured look of blenders.
Also I tend to buy larger quantity than needed for any project I do so there's almost always some left over to use later.