I think each person has to determine how deeply to put their rhizomes in the soil through some trial and error. In my garden I've discovered over a few years that they grow best if buried 1/2 to 2/3 deep into the soil because of my very fine, silty soil that does not hold water very long. I've tried them on top of the soil, too, and this just seems to work best for me -- plus I even add mulch over the summer. But then we get up in the 100's with very intense sunlight in the summer months...
I dig a hole, mix a handful of bone meal in, and then I build half of it it back up to ground level and put the rhizome on that ledge and let the roots hang over, then fill the rest of the hole. And I compact things a bit, too, since our soil is so light that if I don't use pressure everything will surely sink too much later on.
