Ive read that you should plant below the graft for some plums in cold areas (by a neighbor, and Ive seen a thing or two online as well). Generally speaking though if you plant the graft you will more then likely either kill the tree (less likely) or get the tree to root above or at the graft, making the root stocks qualities moot (more likely to happen).

Some people do prefer to grow their fruit trees on their own roots, so i guess those people may suggest doing so.

The best advice and most common is plant at the the same depth as it was in the pot. If its bare root or burlaped, there will be a noticeable darker bark above the roots where the original soil line is.

Form my experience planting depth will depend on your soil conditions. Heavy clay soil seems smother so I plant at root level or make a big mound. What about seasonally high water table?...Again I plant on a mound. Really nice light loamy soil on a slope, I have planted deep and below the graft somewhat and have had no problems(with plums) and no sign of the scion rooting as of yet(some of the trunks are nearly 4 inches in diameter).
All that said I prefer to plant with the roots just below the surface as any wild trees roots are just that. Let them decide how deep they want to send their roots down!

Trees tend to settle after transplanting, so it is hard to get the depth you want without some experience in your soil. I saw or read that digging the hole 3x wider, but no extra dept vs the root size, will get best root growth. that would also help with settling.

A big problem I see sometimes is trees you get from the big box stores. Most people know nothing about the rootstock and just plant them at the level of the tree in the container and many times I've noticed the root stock buried in that container.

What does the nursery say where you bought it if local? Maybe in Canada some Euro plums indeed do better buried slightly under the root flare or even the graft. Pecans are the only trees I know of with good scientific evidence of increased success in planting that way. But I live in a very warm climate and am solely interested in fruit/nut bearing plants. Local experience should prevail.