With regard to the PSU and cahrged capacitors. Actually there are only two capacitors in a PSU that can ruin your day or possibly your life.
They look much like this:
Note their rated working voltage of 200 Volts. Now here is how they look in an actual PSU circuit, this is just the front end lines side with the rest chopped off as it would break tables.
PSU Front End:
The pictured capacitors are actually labeled C5 and C6 in the schematic drawing. Note they are in a series configuration of sorts. Each cap will charge to a peak value of the mains voltage or as configured a peak value of 120 X 1.414 = 170 Volts or combined about 340 Volts.
Now note two resistors beside those caps labled R2 and R3. Those 220 K Ohm resistors serve as Bleeder Resistors" to discharge C5 and C6.
Anytime the PSU is plugged in and the PSU power switch is ON that front end has charged capacitors. When mains power is removed the discharge path for the caps is the bleeders.
Actually those two caps discharge pretty slow even with bleeders. How quick? Calculate the RC Time Constant. Roughly in less that 20 min.The rate of charge and discharge is log and not a linear function.
Note just beyond the bleeder resistors two lines drop down off the picture. Those lines feed the supply for the 5 Volt SB (Stand By) supply which is necessary for the PSU to start. The front panel push button uses that power to start the PSU.
Actually once mains power is removed within a few seconds pressing the front panel power button won't do anything.
I guess if we were really curious we could chart some of this stuff? Might be interesting?
Generally speaking it is safe to work on a PSU despite claims otherwise. However unless someone is real comfortable in there caution including over cautious is the best road to take. Hey, better to err on the side of safety that have a shocking experience.
Edited to fix my suck math!