https://www.starkbros.com/growing-guide/printed-guides/category/14
“For standard-size peach trees, situate the bud union 1 to 2 inches below the soil line.”
Here in zone 4, we are encouraged to plant tender things (like peaches) with the graft union buried. With cold and short seasons, there is already a tendency to lower vigor so size can be controlled by summer pruning and heavy bearing. Having the union buried and forming it’s own roots can gain you that extra 0.5-1 zone hardiness in a test winter which is a big deal when we are already pushing the line. You lose any disease resistance of the rootstock though.