Derrick trucks should always be grounded whether the line is live or grounded (should be the only options) and to the system neutral if available. The reason for grounding RBDs during dead operations is to bond the boom in case of accidental energization to keep it at the same potential as the worker in the air. When working live it is to help clear the line, but the reality is if you touch the truck at the same time as it is accidentally energized you will be badly hurt if not killed.