The graft needs to be well above the soil line. The whole concept is about the rootstock providing a better root system.
As far as planting tomatoes extra deep, I don't plant any deeper than a few inches. Our soil is too cold and the growing season is too short. Some years, the tomatoes are transplanted in June, depending on the weather. Our nights stay very cool throughout the summer. We have about 3 months of growing for tomatoes, barely enough time for the long maturity varieties.
I had the privilege of attending two presentations by Jeff Casey (local Heirloom Tomato Grower) and he confirmed the same. Too deep in our area will not be beneficial in our area.
