I am not sure about this particular board but virtually all other i/o devices I have encountered have pull-up resistors on the input lines. This should be specified in the board description. So external pull-ups usually are not required, and you can leave the lines unconnected.
However, due to high input impedance (even with a pull-up) these lines may pick up noise and may accidentally be pulled down to the low level. If you want to make sure to avoid this, either tie all unused input lines to GND or use pull-up resistors with rather small values (in the 1 kOhm range).
