I, too, have "heard" of that 12 inch limit. That's 6 inches on either side of a full loop. This is close to the 5 inch limit you mention. Long rack runs will have anchors along the way to stablize the line. The spacing of these anchors will break the long run into shorter runs, each which grow no more than 12 inches. The point of 5 or 6 inch growth is to keep the support (and pipe) from clashing or falling.
Again, that's what I've heard.