The secret to a good usable stash is how much to buy of each type of fabric. The largest size quilt I make is queen size so I always keep that in mind.
Most queen size quilt patterns call for 3 to 4 yards for a background fabric if it has a background. 4 to 7 yards is a good amount for the focal fabric since you'll probably want to use it for borders too. And then you'll need supporting players, good tone on tones, small patterns and geometrics and you'll usually use two or three in a quilt so 2 yards is a good amount for them.
I've started buying two extra yards of the focal fabrics so I have enough to make pillow cases too.
Then the quiltovers get saved and are the starters for great scrap quilts.