It is surprising, but I get asked this question quite a lot.
I think I can understand why, when you see on the TV the gardening presenters making a song and a dance about that you should plant to where the soil level was originally.
With regards to most grafted fruit trees this boloney.
As long as the graft union or joint between the scion and rootstock is above soil level then that is all that is required. If growing apples on MM106 rootstock, then should always be planted with the graft union as close to soil level as possible. This is due to excessive suckering of the rootstock.
However there are certain circumstances that this not the case.
This is when you want a variety that is shy rooting to gradually root, with the rootstock then gradually dieing off. This is the case with Medlars. The graft union is buried deeper, so the original Medlar starts rooting and the rootstock which is normally Thorn would die off.
There is also a group of growers who use ‘Own Roots’. Whereby the same method is used to get apple varieties growing on there own roots. Due to the fact that most apples are shy rooters.