Reminds me of the time a fellow I know was changing the battery in a piece of heavy machinery. He was tightening the + lead down on the battery after he had already done the - terminal. Since he was a lefty he was using the wrench in his left hand and his wedding ring made contact with the chassis. The ring welded itself to the chassis and wrench in an instant. Also in that instant the ring became red hot, burning a complete ring around his finger before he was able to yank free. He spent several months not knowing if he was going to loose the entire finger due to circulation problems from the deep burns completely surrounding the finger. If he had reacted any slower the battery would likely have exploded which would have lead to other problems.
As for where to connect the neg lead, most commercial two- way radio installers use the chassis as do a lot of stereo installers. It works well. I myself prefer going back to the battery but that is just because any chassis bolt I have seen tends to rust over time up here in the salt including the vehicle seats simply due to the salt off your boots etc.
