The ic fan on my 90 runs with key on. I put the fuse for it up by the battery, so when the ambient air drops in october and it's 40f in the morning, the fan doesn't need to be running I just pull the fuse.
For drag racers, I'd wire it up to take power right from the battery, and the relay is switched from inside the car also ganged into the ignition. Using a double acting switch, you could have the fan come on with ignition in one position, and then with the switch in another position, it runs with the ignition off, i.e. cool down after a run down the track.