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I beg to differ, both are of import. Depending on the circuitry, the accepted norm is +/- 5% 5Vdc and has been since the TTL days -now with 3.3Vdc circuitry things are a bit different, but if the device expects 5Vdc it may be picky about not getting it even if it is being regulated down to 3.3Vdc or less for the circuitry actually doing the work. Best to always stay within the specs for voltage and meet or exceed the demand for current supply.
Undervoltaging has the nasty habit of drawing excess current in certain devices(most!) as power, the product of current times voltage is the true constant. Said excess current occasionally leads to burnt out traces or other conductors if the PCB designer did not leave enough width(call it overhead or tolerance) and thereby current carrying capacity. Ask me how I learned these lessons low so many, many moons ago... and I'll steal a line from a certain red shirted character(ha, was he ever!) with a scottish brogue who at least once opined that a good engineer is always more conservative on paper! ;-)
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