Generally, we see a recommendation in the datasheet of micro controller to tie the unused pins to a known state. the known state here meant either power rail or ground. Why should unused pins of a micro controller must be tied HIGH or LOW?
The I/O pins of a micro controller has pull-up and pull-down options. If we look at the micro controller or processor I/O pin internal circuitry in detail, there shall be a transistor used to select the option of pull-up or pull-down. When I/O pin is not connected in your circuitry, the pull-up or pull-down are disabled, which meant, there can be chance of capacitive coupling of noise from these transistors to the I/O pin. So, the I/O pin instead of being in static position gets noise coupled. To eliminate this scenario, the pin must be pulled externally high or low. If not pulled HIGH or LOW, the noise coupled to the I/O pin can propagate to the entire circuit or couple to the adjacent pin. the preferred option is to pull down the pin to ground. One has to remember that this can be done by pulling the I/O pin to GROUND directly or having a pull-down resistor to GROUND. It is preferable to pull-down the I/O pin with a low value of resistor so that very less noise gets coupled if any from the transistor circuitry. Adding very high value of resistor to GROUND can be useless as this leads to a increase in impedance and more noise coupling from transistor.
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