burying the graft negates the qualities of the rootstock (ie dwarfing, disease resistance etc.) I plant at same level as it was in the nursery (there will be a visible line with different shaded colours of bark…so pretty much like any other tree.
My understanding is that interstem is kind of special in this regard. There are two grafting points, A between the root stock and the interstem, and B between the interstem and the cultivar. Generally you dont want to burry the grafting point between the rootstock and the cultivar because rootstock usually dwarves the tree so y oudont want more vigourous roots of the cultivar to happens.
Here if one burries the grafting point A between the rootstock and the dwarfing interstem, and that the interstem roots in the soil, it is not a big deal because it wont brings more vigor than if it is not rooted, as the vigor is provided by the rootstock. You definitely dont ant to burry the grafting point B as it will negate the dwarfing effect of the interstem.
Besides, burrying the graftin point A could have benefits as it will make the interstem less prone to breakage.