You don't want to jump start another vehicle with a dead battery because you stand a good chance of ruining the voltage regulator in your alternator.
I think this is a old wives' tail started because some one didn't do it properly. they even explain how to jump another vehicle in the owners manual. you just hook the ground on your vehicle to a ground, not the battery directly. in my years I haver been jumped and jumped hundreds of times and never had an issue.
I have burned out an alternator twice jumping very dead batteries. The difference in conductance between a frame ground and the battery terminal is very miniscule. I will stick with engine off to protect the alternator.
