I NEVER buy exactly what I need - shoot, half the time I don't know what I need. So a yard is a good start for me. hehe
DH was with me the one time I just needed a bit for an art project. When we left the store he said: "Honestly, you can buy less than a yard????"
Regardless, washing may (and usually does) shrink a bit, and not all cuts are perfect. So that exact cut will be a bit short for sure.