I generally buy yardage because I often mix different lines of fabric instead of just sticking to one line for a quilt. I will buy precuts for some quilts though where the patterns has you use 5" (charms) or 10"(layer cakes) squares because I'm usually going for quick when using those patterns! (ok, maybe a bit lazy too!).
I rarely buy FQ unless I'm planning a scrappy quilt and want that mix--I think they are rather expensive and somewhat limiting on what you can do with them--but they are good for applique.
I'd suggest you don't just go out and purchase fabrics for a stash, but purchase for specific projects--I usually will buy a little extra "just in case" and that then becomes part of your stash--and it will grow pretty darn fast as those of us that quilt a lot will attest to!