Grounding unused leg of bussed resistor?
Leave them disconnected. If you connect them to ground, and your input is >0V (with respect to ground), you will be creating unnecessary current draw in your circuit.

Note that this is different from having unused pins on a microcontroller, where you might tie them to ground (or Vcc, depending on configuration) to prevent undesirable side-effects when reading values from an ADC, getting random resets on a reset pin, etc.