PS: And yes, like you my favourite way to power a standard arduino board with an external regulated +5vdc source is to take a spare USB cable, lop off the PC end and extract the power and ground wires and wire that to the external 5 vdc power supply.
I'd certainluy go along with that as a recommendation. The few times I use an actual arduino (as I usually have my own design going) I am powered via PC/USB cable for software download/test.
In my designs, I bring in 5V from a walwart to power the card. Or 12V if LED strings are being powered, and use a 7805 regulator.
Or I bring in power via FTDI Basic for bootloading via MKii.