[SPOILERS] 5x04 Episode Discussion (airs 3rd May 2015)

But the point is that we've never actually seen King's blood having a real value. We don't know if the leeches had any effect on the three kings deaths (GRRM cleverly made it so that we can understand whatever we like from it), nor do we know if the sacrifices of Mance's baby, Joffrey, Tommen, or any Targaryen would have any power. We don't know it, that's why there are no rules. There might not even be an effect; it is still an open question.