If i know what I want to make, I buy yardage. If I am buying for stash, I buy precuts. I like fat quarters and charms. Charms are good for pictorial quilting, when you just want a little of each color. Fat quarters are good for making blocks- you can get more cuts out of them than a regular quarter yard. I only buy them from places that don't charge for the cut though - I won't pay a yardage premium on them. I also buy a lot of quarter yards.
When stash building though, I look for good sales.