[dmjlambert]

It won't hurt to add the resistor. Between 35 ohms (5-Watt resistor) and 500 ohms (0.5 Watt resistor). Since 0.5 watt resistors are much smaller and do not get very hot dissipating 0.5 watts of heat and are less expensive, I suggest something around 500 ohm 1/2 W resistor. 470 ohms is the standard and common resistor value that is near 500 ohms.