I suppose that bonding the drain pipe isn't a bad idea. This would even be desireable in the case of a sump pump, or a 'pump-up' toilet, or other electrical equipment being present that uses the drain line.
In that case, I would prefer to make my bond at the equipment, rather than to a main line. Even cast iron pipe is assembled in a manner that may provide a poor connection between separate pieces. So let's put the electric "drain" near where the electric "leak" is likely to be!
