Actually, a detail about your comment on 12Vdc supplies.
Probably still a good idea to respect good electrical practices, but if you think about it for a moment, a human body is actually quite a low electrical load (compared to the windings of a starter motor for example) so the effect of touching anything on a 12V rail is like connecting a high ohm resister in parallel with the circuit. 12V across this 'resistor' would not result in amps in current. It may still be sufficient to shock you - it all depends on what your body's resistance works out to be AND the contact points (for example, most dangerous ways of working is to lean above a circuit resting on one hand and probe around with your other. You have effectively used the path across your arms and heart to create a 'resistor' connected between 12V and earth... if that results in a few 100mA current, you're in trouble
Things you need to take more care of probably are, any large capacitors that might have a stored charge (because that's where you might have a high voltage source you weren't aware of that for a very short duration could result in a substantial current through your body to shock you).
Makes sense?
