If you mount the axle under the springs it is completely unrestrained in case of U bolt failure. If its above its at least contained by the springs and hangers. Ill be keeping mine above
Axles have been usually over springs on all cars, trucks and trains since adam (and I) were a boy.
Prpbably some other techical reasons as well (bump steer included).
Cheers from a newby.
Al.