Normally as long as the variations are less than 10% then you are fine. The max that the 5volt can fluctuate for our example, is like to 4.85, to 5.5. A little lower or higher than that is fine depending on what it starts at. A good rule of thumb is to not let the voltage rails fluctuate more than 10% from where they started at. Speed fan and motherboard monitor 5 will both tell you your temps and voltages.