With the first car I built, I had the switch mounted on outside of the car with the thought process that the corner worker could easily reach it if need be.

I got into an accident at Lime Rock, and found myself perpendicular to traffic a little ways after the crest of the downhill where cars couldn't see me until the last second. There was steam (hard to tell at the time and circumstances if it was smoke). The corner workers could not get near me for what felt like a long time do to where I was positioned. The whole time I kept thinking about how I couldn't reach the kill switch.

For me, first priority is that I can get to the switch and turn it off. I've been in enough incidents to know (at least for me), this is the most likely situation. If I were only to do one switch, it would be on the drivers side roll bar where I could reach it as well as a corner worker.

For the kill switch itself, if you don't use Stephen's method, get the heavy duty metal kill switch instead of the crappy plastic one. I've had those fail too often.