The 5v/12v are most important, especially the 12v. You want those voltages to be either at, a little below, or a little above.
The 12v rail is the lifeline of all modern day systems as it powers the main components such as your CPU/GPU.
The 5v rail powers USB, PS/2, PCI headers, storage devices, etc.
When the voltage is super low on one of these, (9/10v instead of 12 - 2/3v instead of 5v) your PSU is faulty as it's not supplying enough power to your components, and you'll get crashes, freezes (mostly), etc.