I find that if I only purchase fabrics I really like, ultimately most of them will play well together. Also, I treat my stash like my own private Quilt Shop. I do have some fabrics that are waiting for that PERFECT pattern (for them).
That being said, occasionally, I purchase a fabric that is just not me -- those go into pot holders, pincushions, sample blocks -- someone will really like that fabric -- even if I don't remember why I bought it.