You might consider using more than 80 ohms, especially while you are testing. This way you can ensure you don't accidentally have a setup where you Source or Sink >20mA from a single pin. Another number to keep in mind is the absolute max current rating for the VCC and GND pins. The max is 200mA. Which means you probably don't want it to be more than 150mA.
Your comment: "The thing I am concerned about is, surely using a low pin as a ground pin will pour in power that is not neeeded or wanted, and wreck
havock with the inner workings of the microcontroller?"
I'm not sure what any of that means. When a pin goes "HIGH", it is connected to VCC through a transistor. When a pin goes "LOW", it is connected to Ground through a transistor. Look at the functional diagrams for an i/o pin in the datasheet. What is the source of this "power" that will "pour"? If you Source or Sink 40mA or more though the pin, you are likely to burn out the pin's drivers. There isn't much else to wreck.