You can start quilting at the outside edges if your quilt is glued. In fact, I recommend it.
Starting at the center is the right way to go if your quilt is basted traditionally. But if it is glued, you want to stabilize it from the edge first. The pressure from working with a large quilt sometimes causes the glued pieces to give way at the edges. The last big quilt I made and had glued, I sewed a line all around the outside, then put the binding on right away. Then I started at the edges and worked my way to the center doing SID. Worked beautifully.