Should a diode and/or transistor be used with an automotive (Bosch style) relay for accessory power?
You should put one in though it is not as critical in this case since the a manual switch is a lot hardier to voltage spikes than a transistor which is often the first thing to blow from the spike. Even so, having one will prevent the switch from arcing and wearing it out.
You should put one in anyways just so the spike doesn't affect other things connected to your battery. Place the diode parallel to the relay coil in the direction that does NOT normally conduct current and short the battery out.