The more accurate the PSU is, the more stable your system will be. I've never seen a PSU that was exactly what it should be. Generally, the weakest line is the 12V line. This is a problem, however, since almost all components in the system use 12V. Most importantly, most CPUs now require a heavy load on the 12V rail. Thus, a bad 12V line can have dramatic effects on a system.
The most important thing is to know the amperage on your 12V supply. The higher the amperage, the more stable the system will run, since you won't be pushing the PSU to supply pwer at its absolute limits.
Most PSUs are specd to run within about 5% of their respective voltage levels.
My system runs:
11.85V on 12V (with 20 amps)
4.93V on 5V
3.36V on 3.3V
my CPU is under-volted to 1.575V (from 1.6) just to decrease the temperature a little since it was running warmer than I preferred.
My PSU is an Antec SL450, and it has operated flawlessly. So, the moral of the story is that more accurate rails will net a happier, more stable system, but even high quality PSUs won't always operate at full voltage levels. As long as they are running within a few percent of maximum levels, however, I've never had stability problems.
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