Reduction of DC motor noise
You should always put a capacitor across the motor terminals even if your circuit is not affected, because brush arcing creates rf noise that can interfere with other equipment (eg. AM radios). The usual recommendation is to install two 0.1uF ceramic capacitors, one connected from each motor terminal to the case. This 'grounds' the case to rf without the danger of having an exposed DC connection. 

Ripple can be a problem for sensitive equipment which has poor power supply rejection, but normal filter capacitors and regulators will usually eliminate it. Another concern is the current spike and voltage dip that occurs when starting the motor. This motor has a stall current of only 390mA, so provided your 12V supply can handle that you shouldn't have to worry about it. Just make sure that the motor and its control circuit is wired directly to the power source, and run separate wires to the other devices.