You want the tree to be planted at the same height it is in the container or where the soil begins for a balled and burlapped tree. Sometimes, however the tree's natural depth gets significantly changed when it is repotted or dug. As a result, occasionally a tree is either planted too deeply (more common) or too shallowly when you purchase.
Look at the flare of the tree (the flare is where the trunk widens as the trunk goes into the soil). You should be able to see some flare, even in a young tree. If the tree trunk goes into the soil absolutely straight, like a lollipop stick, it is buried too deeply in the pot. Scrap away soil at the base of the trunk until you see a widening. Plant it at that level.
Our website pub on planting trees is very good and helpful: http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG24_Planting_tips_f...