There's at least the 5V standby in an ATX system when it's shut down. The ATX switch does not really do much, merely a signal to the motherboard to connect the "pwr_on" cable of the PSU to get it started.
So, technically, it's never really off.

5 V won't hurt you but they're enough to damage a part if you short something.
I use the external mains switch on the PSU before I touch the insides.
If the PSU does not have one, you should pull the cable.
Plenty of alternatives to discharge static. Most of the times, touching the case steel will suffice.

I'm a bit confused by your reply, but regardless let me just cut to the chase. I have an old serverboard with a 24 pin atx connector, but the voltages are likely different. I plugged it in, no cpu's or memory iirc, but whatever, and flippa da switch. Did I fry anything? I now know where the mobo power pins are, but didn't then. Pin 9 is 5vsb on the p/s. Pin 9 on the mobo may be 3.3vdc. Not positive of that even, but what if?

There's an ATX entry on wiki including a power supply section - have to look that up myself all the time.

So, you'll need to know the basics of that and a manual of you mobo, before you touch anything.

General rule, before you open any electrical appliance, detach the mains cable.
The little switch outside on most ATX PSUs is merely a more convenient way to the same effect.
And, iirc, that should cut both sides of the mains cable, meaning "life" and "zero" but keep the "earth" connected.

Ok I wasn't careful with my terms. This has nothing to do with the innards of a p/s. Only voltages present at the 24 pin atx connector. So sorry.

sure, I assumed as much. Hence the "general rule".
The PSU itself, you should not open unless you have professional training.

Cuttoon wrote on 2022-03-30, 22:15:

sure, I assumed as much. Hence the "general rule".
The PSU itself, you should not open unless you have professional training.

That's a bit over-dramatizing it. There are some big caps in there that can store a dangerous charge for a time. Discharge them by shorting the pins for a few secs and there's not much to worry about inside the PSU. Compared to a CRT it's straightforward.

Having said that, someone not sure about +5VSB probably isn't the best of people to start messing around inside PSUs.

OP, if that's an old (pre ATX2.2) server, that 24p connector may not be regular ATX. If it's a Compaq server, they had their own 24p connector around the turn of the millennium. If it's one of those the pinout will be different and needs looking up. You can find a lot here: https://pinouts.ru/Power/

Without studying it intently, this (https://old.pinouts.ru/Power/atx_ges_24_pinout.shtml) could be similar. Thanks for the link.

Re: power for an old serverboard