The capacitors within your psu will drop fairly quickly. You have to remember, in an ATX supply, there is a 5v constant, always draining current from your house. So, when you unplug it, that 5v regulator is gonna burn off those big caps fairly quickly. If you are THAT worried about it, take a meter and test the 5vsb lead against a ground or the casing of the PSU. If it reads 0, then they are discharged. Plus, even if they are fully charged, its 35v DC current, which wont shock you, it will just tingle A LOT and kinda burn your skin. Not that I'd know or anything. Yeah, and dont lick em either
they taste sooooo goood!!!