A Nano has a 500mA schottky backflow protection diode between USB supply and the 5volt pin (no polyfuse).
That means that you can safely connect 5volt to the 5volt pin when the USB lead is plugged in.
A Nano runs on 5volt if you supply 6-9volt to the V-in (raw) pin. Or 5volt to the 5volt pin.
But on USB supply the 5volt pin is about 4.6volt, because of the backflow diode drop.
Leo..