i do have an extensive stash---i (generally) decide everything on a very individual basis---if it looks like it would make great borders- or is a border print i usually will pick up 3 yards so i can cut borders lengthwise=not have to piece them.
if i love it & think it would make a great backing or background i will buy 6-10 yards (or the bolt)
if i like it but really don't know what i would do with it- just want a piece of it that may come in handy i buy either a fq or 1/2 yard. when i look at any fabric that (catches my eye) i generally immediately start envisioning the possibilites---lots of blocks, appliques, borders, backgrounds, sashings, a blender that could be used over & over again in numerous projects....then i decide how much of it i think i should get. i very very seldom purchase pre-cuts or anything less than 1 yard-