I usually have a 'to do' list of 3-5 quilt patterns I want to make. I check to see what I have in my stash, and what I need. Then when I see fabric on sale I know what I need for any of my 'to do' quilts.
I keep these stacks separate, along with the pattern, so I know when I have enough fabric for the quilt.
Solids and tone-on-tone fabrics are probably what I stock up on when it's on sale whether I need it or not.