12volt should be within 11.4 - 12.6v
5 volt between 4.75 and 5.25v
3.3 between 3.13 and 3.47v
DO NOT rely on any onboard voltage monitoring system. They are notoriously unreliable. The only way to know what your actual rail voltages are is to measure them with a digital multimeter (around $25 at Radio Shack).
Keep in mind that most high-end PSUs nowadays have dual 12volt rails so to measure the 12volt line you need to measure the voltage seperately at the PSU mobo connector and one of the HDD power connectors (12 volt is measured across the yellow and any black wire in both connectors).
Unless you're having mysterious crashing problems, I wouldn't worry too much about your voltages.
Hope this helps...