Just because it's DC doesn't mean you can't get elecrocuted. Your body's skin just has a high ohm resistance, so that 12 volt would end up feeling like .6 mA, which is hardly felt. But if you get under the skin (stab yourself on the car frame and touch the pos of the battery) the ohm resistance is much less and you can get a nasty shock.
It's also best to play it safe. And besides, you're helping to protect your equipment the most.