David said:
Yes, every CMOS input should be tied either high or low, or driven by
another gate, your pick, just don't leave inputs floating.
Nobody explained why there is this rule so for your information: CMOS
inputs are VERY high impedance. Therefore if left floating they can
pick up charge and develop some voltage. The problem comes when they
develop a voltage in the middle of the logic range. In the middle
switching range the current through the transistors in the given gate
tend to draw much more current than when the input is full high or
full low. Therefore the distinct advantage of CMOS using little
current in static cases is lost. So if, say, you were developing some
low speed battery powered logic, some floating gates could very
drastically shorten battery life *even if those gates were not
connected to anything but power*!