i always use the white wire for the kill switch. i know there's been some posts about whether it's good or bad, but on all the bikes i've built i've never had a problem.
two of my bikes have the kill switch mounted in the layback seat post so it looks like i'm scratching my butt when i kill the bike. one has an on/off toggle, and the other is just a momentary button.
i drill a small hole in the bottom bracket, away from any welds or anything that could cause stress or a breakage, then run two wires up to the switch. one is soldered to the white wire and the other goes to the ground, which i ground to the rear fender stay or the kickstand bolt.
there's a coupla pics on this page:
http://motorbicycling.com/f38/one-more-40s-colson-add-fleet-21037-11.html
as far as the safety issue goes, the only time i think i'd have to shut my bike off in a hurry would be if it's on fire. even with a stuck throttle, the brakes would still probably stop it. it's not heavy like a motorcycle, so it'll still slow down pretty quick. you'll also get used to shutting it off, so if you have to sneak up on someone, it's no big deal, as long as it's accessable while riding.