Assuming that your theory that the wildlings left north of The Wall were those who condoned and practiced human sacrifice to the White Walkers, through what sort of mental gymnastics would these wildlings have gone through to justify human sacrifice, given their disgust for incest and cannibalism? It seems to me that human sacrifice rather goes against the rough structure of morals that they otherwise lived by.

For Ygritte’s tribe, and we don’t know which one it is, it’s custom that “women who bed brothers or fathers or clan kin offend the gods, and are cursed with weak and sickly children.” But it’s custom for the ice river clans to practice cannibalism, and as much as Ygritte argues that Craster is more Night’s Watch than wildling, that smacks more than a little of No True Scotsman. 

If my theory is right, I would imagine that much of the memory of the original sacrifices has been lost to time, and indeed may have been deliberately erased from oral histories in the same way that the Night’s King was erased from the written records of the Night’s Watch. (Wouldn’t you if your people had done something so heinous that the rest of humanity exiled you behind the Wall?) 

If it’s done at all today, it probably follows the model at Craster’s Keep: a largely unspoken and uncertain practice (Craster’s understanding of what makes the cold gods go away is pretty ramshackle – hence the missing sheep) carried out in isolated communities that have always done things this way, out of a vague sense that somehow it wards off ill fortune.