You want to plant ANY tree - grafted JM or not - so that the root flare is at or slightly above the soil surface. The root flare is that portion of the tree where the trunk begins to spread out to form the root system. Since any graft union will be well above this point, at least by several inches, there should be no chance of this being buried at planting time.

Generally, burying a graft union - other than resulting in the plant in question being planted too deeply - will cause excessive growth or suckering of the rootstock, which has the potential of overwhelming the scion or desired grafted top growth. It can also promote graft failure.