At my stage in life, (I do have a large stash) I usually only purchase something on sale or if I need a fabric to fill in what I do not have, then I buy. I do sometimes buy a full bolt of black or white or cream as I use lots of these neutrals. The white and cream I dye to the color I want.
Buy what you can, when I was building a stash, I bought 1yard of fabric I liked, accents I bought fat quarters. If I thought it would be borders, 3 yards. For backings I would buy on sale. (still do that)