If you haven't already figured it out, (this thread is a couple of months old), it isn't the voltage so much to be concerned about...it's the current. If the supplier said that they will work fine with 12 volts then that may be the calculated voltage for the supplied resistor. Resistors limit the current to the LED so that too much electricity does not flow through it. Definately run the LED's in parallel and definately use the resistors (on the positive side). The resistors supplied with your LEDs are obviously meant to light the LED with a 12v source. However, if you ever decided to use a different voltage, lets say 6 volts the calculation is fairly easy. It's just Ohm's law R = E x I. Where R = resistance, E = Voltage, and I equals current in Amps. So....if the LED was rated at 50ma or 0.05 amps, the voltage rating was 3.3 volts, and the supply voltage was 6 volts, the equation for figuring the resistance is (6v - 3.3v)/0.05 amps = 54 ohms. A resistor with a value of 54 ohms would safely light the LED. The reason for doing this little calculation is not only to safely limit the current through the LED, but a resistor which is too large will dim the light too much and not be good either.