keep surface roots buried until they are fairly thick. When you repot, you arrange surface roots, rake them out, like spokes of a wheel. then bury them at least 1/4 to 1/2 inch deep. You don't expose the surface roots until they have bark, not until they are the same diameter of 3 to 5 year old branches. Usually it is safe to expose surface roots when the tree has been living in a bonsai pot 5 or more years. Expose the roots too soon, they dry out and die. Then you end up with the tree sitting on a raised cone of roots, and often not attractive roots, as the only roots that survive are the ones that go down.
Some people like these raised volcanoes of roots, but a flat, radial root system is much more refined.