Over time Howard has looked at a lot of trees that aren't thriving, and it became clear that they were planted too deep. If you have to fault on too deep or too shallow, I think too shallow is the way to go. The root flare should be exposed. Potted plants are usually already too deep, and bare root looks like the best way to plant - but don't lose your advantage by planting too deep!
Here is the guide that gives instructions on how to plant trees: http://www.dirtdoctor.com/Tree-Newsletter-Planting-Series_vq1941.htm