In most PC build guide videos like the ones in Newegg TV, they recommend you to do a pre-build and boot to make sure every essential component is working. Is it safe to turn it off directly by switching the PSU button or just unplugging it directly from the wall socket?
What motherboard are you going to use? Some have onboard on/off/reset buttons. Also, you could hook up the power button of the case if the cable has enough reach. Activating the powerbutton pins on the motherboard with a jumper will also work.
EDIT > and using the switch on the PSU will work too. Bit crude, but this should be no problem for this one time ocassion.
The BIOS can be exited pretty much anytime by cutting power (no permanent damage to be done here), but to be sure just press F5 (discard changes and exit) and Enter (to confirm) and then shut off power.
just for the love of god don't turn off while flashing the bios and you will be fine :D
No, don't worry about it, even with an os installed or not. Computers these days can handle power outages no problem. Anytime something turns off all its doing is cutting power.