I only follow my own rules for border size. For me, if it is a couch throw, I use either no border, or very narrow borders if the pattern needs it to look complete. For most bed quilts, I make the center that just covers the mattress, then the borders are how much drop I want on the sides. Some patterns still look best with no border though. So no set in stone rules for me. Wall hangings, anything goes as long as it looks good.