kd7eir:
If the voltage is regulated to EXACTLY 5V that is true, but I would not trust any power supply that did not cost several times the price of the Arduino to maintain that EXACT 5V level.
Well, the 328 is rated 6V "absolute maximum" as I recall, so it is not really that critical. 5.5V should be a safe excursion.
I do however think that if you value your circuits, a 5V (switchmode) supply should have a crowbar.