It really depends on the fabric/project.
For some, where I have a very specific project in mind - like the baby quilt for a Duke fan - and I know I'll never use the fabric again (rabid TarHeel fans here), then I buy as close to what I need as possible.
For others, like Laurel Burch fabrics that I love - I try to buy a yard or two at a time. But I'm also finding that they sit while I try to find a pattern that will fit them. So not the best use of limited funds.
I think I average 1/2 yard to 1 yard if I have a rough idea of what I'm going to do with the fabric.
I like fat quarters when they are on sale or packaged to be less than $2.00/yard or less than 1/4 of the price of that particular fabric/yard. I haven't bought honeybuns or jelly rolls yet - in part because I do like duplicates of patterns - so getting just one or two strips of a given fabric is harder for me. My LQS puts half-yard cuts in baskets rather than FQs so I'm more likely to buy in that length for impulse buys.
Cheers, K