I am wiring up a kill switch, which will use a relay to connect/disconnect the neutral safety circuit. Would there be any disadvantage of powering this relay from an ignitiion switched source, so the relay is always energized during normal driving? It will not be handling any current except for during cranking.

 

I don't think I would wire it like that. I would go with a good, quality switch that is rated for the current during cranking. I try to only use relays where they are absolutely necessary. A switch is much simpler and more reliable, in my opinion.

I like the idea I heard about a long time ago where the switch is a push button type mounted in the bottom of the cigarette lighter. If the lighter is pulled, the car won't start. You have to push the lighter in to make the switch and get the starter to engage.