If I find fabric that I like, I buy it. I have sort of a rule that I will only buy the fabric if I can find two more fabrics at the same place that I like with it. The rule hasn't served me too well, but I use it as a guide line if I am not sure I love the fabric, but I would still like to have it. Basically though, if I like it and want it, I buy 2 yards. If I think it might end up being binding and a border too, I buy 3 yards.
My stash is small by many standards, but I do like being able to look at it and start a quilt without having to go shopping.
Dina