There certainly were no quilt police when they wrote that book. The guild I used to belong to refused to allow tied quilt in the yearly quilt show. One visiting quilt teacher, who was there for this discussion, pointed out that a recent quilt magazine had a tied quilt on the cover, and she would definitely have allowed it in a quilt show. So if it's OK to tie and be on the cover of a quilt magazine - go for it. For some reason, I'm all thumbs when it comes to tying quilts, so I stuck with simple machine quilting. Whatever works for you, just do it as it is your quilt.