Disconnecting a resistor out of parallel with a switch
Yes, the voltage, current, and frequency of the signal going through the resistors is relevant.  For example, if you are simply changing the brightness of an LED, then a SPST switch will be fine.  If you are modifying one leg of an RF attenuator, then a SPST switch will create an unterminated stub that could be a real problem for the downstream circuits.

Without knowing more about the load, this is a guess:

Assuming the motor is DC or powerline AC (50-60 Hz), a SPST switch should be fine.  For better long-term reliability, use a switch with contacts rated for at least twice the expected breaking current.