Since it will be your quilt, you can do it anyway you want to. Mostly string quilts seem to be a way to use up scraps and it is faster to use the machine. I can't see any reason why you would have to start in the middle. Why not try one and see if you like doing it. I started doing English Paper Piecing so that I could sew something while watching baseball - turns out that I enjoy doing it. Be sure to share pictures of your blocks!