I base my fabric buying on two principles: do I have a particular pattern in mind? eg stack and whack quilts take many repeats of fabric so it is no good buying one yard of a fabric if that's what you might like it for; and how annoyed am I going to be if I come to use this extremely unusual fabric and find that I need 1/2 yard more, but it is no longer available?
I learned my basic quilting skills from a dear friend, and I cannot tell you how many times I watched her drive all over town trying to find another 1/2 yard of something because she had only bought 1/2 yard to begin with. Now, whenever I am confronted with a fabric I like but have no immediate use for, I think of her and buy an extra yard!!
Alison