Reading it or not Nigel if the pin is floating the input impedance is pretty high, accidental capacitive coupling to a nearby component can occur which can cause the input pin to oscilate, even if it's not being read the internal comparators are triggering and it draws power, especially if the signal is of high frequency. If you have a ground plain even without physically connecting the pins setting them to outputs and low would be a good idea. I've had pins left floating follow a nearby pin's output voltage on breadboards before.