CD74HC4051-Q1: may I connect the the unused input pins among A0 ~ A7 pins to GND ?

Paul,

Yes, you may connect unused signal path pins A0-A7 to ground, Vcc or floating.  

The note 3 you referenced above is not concerning signal path pins A0-A7 but ADDRESS SEL S0-S2 control logic pins.  The appnote SCBA004 explains how floating in puts to the control logic buffere can cause both the N channel and P channel FETs to partially turn on and short Vcc to ground causing excess current consumption.  For signal switches I've seen this current as high as a couple of ~1mA. 

Thank you,

Adam