[IndyCobra]

Actually, voltage spikes (the perceived need for the diode) are mostly a figment of the imagination. The mostly likely scenario to produce a spike is when the alternator is removed and then added back into a charging circuit when the engine is running. In the aviation community, there's typically a switch that allows the pilot to remove the alternator from the circuit in order to run the electrics from the battery only. This is done for safety and redundancy reasons. Cycling this switch "may" cause a spike, but even that has been elusive to prove scientifically. In automobiles, the alternator is wired to always be in the circuit so there's no way to turn it off and then turn it back on again.

Bottom line is whether you believe in voltage spikes or not, I wouldn't worry much about diode protection in your relay. It won't hurt anything to have it, but it likely isn't doing anything to begin with.