I have never heard of this rule. I am the type of person that wants to know "why" and "since when". I have several vintage quilt books/articles. There are no rules in them. Think of our foremothers quilting way out in their farm houses. How did they learn of the rules? No TV, no computer, not a lot of books (some could not read). Do you think the women quilting in the covered wagons as they traveled West, followed any rules?