I am newer to quilting and these ladies probably know better, but I just did a twin sized quilt with 8" strips for the binding, sewn straight to conserve fabric. I pressed the seams open and made one long strip for the binding. I must of got lucky because none of my seams ended up in corners. If I had more fabric I would have went with diagonal, but as it was I ended up with only 10" extra. After I washed it I did not notice any bulkiness.
Good Luck