The +12V spec is +11.40V to +12.60V (Page 29, Table 23 of the Intel EPS12V Power Supply Design Guide)..If it falls out of range at any time, I would RMA it.

Hello Swash87 and welcome to the Forums.
 
Software level voltage readings can be highly inaccurate. In the BIOS you can check if the PSU is providing consistent power. Go into your BIOS and look for the +12v  reading, this is typically under 'PC Health Status', System Monitor, Hardware Monitor'. A healthy reading is between 11.4 - 12.6v, and stable. It should not fluctuate rapidly more than .05v.
 
You can also use a multimeter or voltmeter for the most accurate reading. HERE is a good link describing how to check the voltages in this way.

Here it is at 11.28v under light to medium loading (standing still in GTA V), jumping quickly between 11.28 and 11.32.

 
Under heavier loads (near 100% GPU load on 2 sli'ed GTX580's) is when it starts dipping into the high 10v ranges :O (ignore the "negative" reading, i've just got the test probes backwards)

Thanks Joseph!

Yes if someone have cheaper multimeters in house, as me  should check at least on two of them.
If both show different results than should be checked again with proven proper multimeter in some service. 
If one show 11.35 and other 11.25 than should be checked again with proper tool in idle and under load.
GPU and CPU stress test in same time. OCCT have some PSU test but should be checked voltage with voltmeters during test.
That's 5 minutes.
 

Yes, I know it is a bluepoint multimeter and I could tell that you were a mechanic based on the picture. I am a mechanic as well. I just wanted to make sure that you verified that the meter is still accurate.

Your mulitmeter will not be able to tell you whether it is fluctuating within or not within the ATX specification. The ATX specification is for ripple current which is a high frequency characteristic of a buck converter. The ripple frequency will be at thousands or millions of hertz and your RMS multimeter cannot detect or display the ripple current. The only somewhat accurate (but still not really at all) approach is to put the meter in AC mode to read the ripple current as millivolts of pulsating DC. For accuracy, you need an oscilloscope.

The only thing you can really go by using a multimetet is 11.4v to 12.6v. Any amount of fluctuation within that range displayed on a multimeter would be considered acceptable.

ATX spec:

Ripple and noise are defined as periodic or random signals over a frequency band of 10 Hz
to 20 MHz. Measurements shall be made with an oscilloscope with 20 MHz bandwidth.
Outputs should be bypassed at the connector with a 0.1 µF ceramic disk capacitor and a
10 µF electrolytic capacitor to simulate system loading. See Figure 5.

Your multimeter does not read and display at 10 times a second or faster.

Additionally, the ATX specification allows for up to 10% voltage fluctuation (instead of the normal 5%) on some 12v DC busses at max load. That means that some 12v DC sources can range from 10.8 to 13.2 volts at max load and still be considered acceptable. And, the -12v DC output to the motherboard is allowed up to 10% fluctuation at any load setting.

ATX spec:

+12V1DC ±5% +11.40 +12.00 +12.60 Volts
+12V2DC (1) ±5% +11.40 +12.00 +12.60 Volts
+5VDC ±5% +4.75 +5.00 +5.25 Volts
+3.3VDC (2) ±5% +3.14 +3.30 +3.47 Volts
-12VDC ±10% -10.80 -12.00 -13.20 Volts
+5VSB ±5% +4.75 +5.00 +5.25 Volts
(1) At +12 VDC peak loading, regulation at the +12 VDC output can go to ± 10%.
(2) Voltage tolerance is required at main connector and S-ATA connector (if used).

Edit: I am not saying that your PSU is not bad. I too would be concerned if my PSU varied so much. I am just saying that there are some weird things that a PSU is allowed to do and still be considered good.