If it is a focus fabric, I always buy 3.5 yards so I can make one piece borders, and use the remaining in any blocks I decide to use.
If it is a complimentary fabric with a design I like, I tend to get 2 yards.
If it is a great backing fabric and on sale, then I buy six yards.
Because I make many patriotic themed quilts, I usually tend to stick to that type of fabric when I see it on sale. However, I am "accidentally" buidling and very large stash of gorgeous oriental fabric, and I hate the thoughts of cutting it!! No design in mind yet, but lots of thoughts, and plenty of fabric to work with! Most are three yard pieces.
I tend to keep my quilts to between three to four fabrics, so and I keep small swatches in my car in a plastic bag so that I always have a sample available when shopping.
Diane