The specifications say minimum is 6V. The reason seems to be that below 6V, the 5V output pin can drop below 5V. But the chips I have hooked up to 5V output have a minimum voltage rating of 3.3V; so a little below 5V is OK for me. Aside from the 5V pin voltage drop, are there any other operating concerns I should have with feeding the micro 5V?
Can damage the regulator.
If connecting power to the 5V header, add a diode from 5V (anode) to Vin (cathode) to avoid reverse driving the regulator.
Or supply 5V via the USB connector.