I usually buy 1 yard pieces of fabrics that call to me. Sometimes I'll run across a fabric that I think will make a great backing and buy 2 yards of it (I make square 48" x 48" quilts). If I'm buying the 1 yard and it turns out that it's the end of the bolt, then I will buy the entire piece. This method of purchasing sometimes means that I'm totally out of a fabric that I've decided to use for sashing and need to run back to the wholesale fabric warehouse where I do most of my shopping. And after I've enlisted the help of the owner, not being able to find it, he then says "why didn't you buy enough in the first place?" and I've had to explain that a lot of us buy fabric with no clue what we're going to do with it.
At this point, it's a darn good thing I have a large stash because I was laid off from my job this week and have been able to spend my time making perfect applique circles for some future project.