Thanks Lefty. I've done a little breadboarding so was aware about the use of a resistor with an LED. I only used Pin 13 as an example because as I understood it that Pin 13 already has a in series resistor with the on-board LED.
Erdin. Thanks. It's good to hear that I won't be putting undue stress on the board by repeatedly unplugging and replugging the USB during my experimentations although your post suggests I should be okay with careful rewiring on the fly? I would only do that for basic changes anyway. If I was making more elaborate changes, then I'd definitely unplug first.
I was just wondering how others typically operate when building really.