Pc voltage

While the spec for the +12V is min of 11.4. I normally ditch my PSU if under load it drops to 11.6.

(1) I Normally always verify my voltages with a DVM so that I know the software is correct. A DVM is only $15->$20 at walmart. They are very easy to use (Just google how to use a Digital voltmeter). Thay also come in vary handy around the House and car.

(2) The +5 V specs are hard specs prior to home computers, it is based on the specs for TTL logic (4.75 -> 5.25 V). The 5 % Is just that standard caried over to the +12 V.
A computer may work fine at say 11.2 V. So Why my 11.6 cutoff - The lower the +12 V is the harder the voltage regulators for the CPU have to work and the greater their heat generation. This is also true for other circuits. Active circuits thend to increase current when the input voltage is decreased.

As to regulation. Not sure if that would apply to all Computer PSUs. If so, pretty crappy idea.
I have seen PSU that regulate one voltage (normally the Higher Holtage) and intern that allows a "lower" voltage to stay with in an exceptable range, based on expected LOAD deviations. The +12 V rail is now the highest loaded rail, unlike early computers where the +5 V was relatively high. A shift of 8.3 amps (100 W) on an unregulated +12V will produce a higher an longer duration change in output "E".