Just as a clarification - the "spike suppression diodes" do not protect the electrical system from high voltage spikes; they protect the switched device, which in this case is the solenoid coil of the relay being turned off, and the switch itself.  The coil really doesn't care, and the switch is designed to tolerate the resulting arc.  The normal wiring (I'm no good at putting graphics in an email so I'll do it "verbally.") Goes from the battery and all your expensive electronics to the switch, to the relay coil, and to ground(return).  The high voltage spike exists only between the switch and relay coil, no where else.  You might be reducing low-amplitude electrical noise in the system being transmitted through the wiring, but you are not protecting the rest of the system against high voltage spikes.