Adherence to National Planting Standards (ANSI A300 Part 6) recommends that all rootball-supporting material (burlap and wire basket) should be cut off from the top third of the rootball.
Generally, the less hindrance for roots growing out from the rootball the better, since a tree only becomes well-established when its roots grow out into the surrounding soil. Also, always remove anything wrapped around the trunk (cord, etc.).