You don't necessiarly need to remove roots, but slice them like Al said. While the seedling is in the container, the roots will grow out to the container, then follow the edge of the container around, making a ball so to speak. If the tree is transplanted and the roots not sliced or trimmed, they will continue to grow around and not out. Then if any wind comes up, the tree will be uprooted a lot easier.
Once the tree is transplanted into the ground, it's better if you water it in a circle at the outside of the branches vs. watering it at the trunk. Roots will grow and search for water, so getting the soil damp out away from the trunk will also help train the roots to grow wide rather than stay growing under the tree.
It's also better to water the tree slowly, for a longer period of time vs. a lot of water at once. That allows the water to soak into the soil to a greater depth and width, making the tree roots follow the water. When the tree is transplanted into the soil, mulch around the tree, but pull the mulch back from the trunk of the tree - that helps keep the moisture in the ground vs. evaporating into the air.
Google "subsurface fertilizer injection trees".
