The reason you don't stretch the short areas of the quilt to meet the wider areas is because those stretched areas will tend to cup (not lay flat) after the quilt is off the frame. This will even happen with an entire quilt when one side of the quilt has a tighter overall tension on the frame than the other side. For instance, if the top of a quilt is very tightly loaded on the frame while the backing is considerably looser, when off the frame the entire quilt will tend to cup like a soup bowl. This sounds impossible, but it really does happen.