The built in fail safe becomes useless in the event of a dead or disconnected battery. The throttle return spring is a must in my opinion. In addition KillerRC.com sells a nitro kill switch that stops the flow of fuel in the event of loosing signal or dead battery as well as temp kill and ability to remote kill the engine via a third channel.if you have the RF-40 rx, it has a fail safe built in,
Fail safes aside you could also get a battery buzzer to use while you drive and will alarm you when voltage drops to a certain level. I have a couple myself and the alarms on them are pretty darn loud so you shouldn't have any problem hearing them.