In 2003 IRC, E3509.7 it states "Where installed in or attached to a building or structure, metal piping systems, including gas piping, capable of becoming energized shall be bonded..."
If the "system" can become energized then it needs to be bonded. I don't see any hard and fast rule, but the reason is there, we just need to apply it.
Lest a newby read this, this is to bond the pipe, not ground the electrical system, totally different purposes.

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Jim Luttrall
www.MrInspector.net
Allen, Texas 75002