CD4066B: About termination of unused pin

Yusuke,

Technically it doesn't matter whether the unused analog inputs are floating or not in terms of functionality of the chip. However, it is always good practice from a design sense to simply ground these. Floating analog input pins can cause voltage to build up, since there won't be anywhere to discharge and could be a concern from a reliability stand point down the line. So while not totally necessary, grounding the unused inputs would be a wise choice.
Since switches are symmetrical, as in each pin can work as input and output, they would be better served grounding both sides. So all unused pins (1-4) to ground. 

Rami