In my opinion, you don't really need a Fail safe, but its good for peace of mind (if your the peace-loving type [I say assasinate Saddam and all his doubles! let's get rid of this war-talk]...anyway
) I haven't used one, but have always used a throttle return spring or two.
The RTR comes with a throttle return spring (I believe), but I like to connect them to the servo horn so that the brake can be engaged just slightly. To do this, I usually take that mount offered in Team Losi throttle return springs and mount it on the servo mount screws that are furthest from the servo horn. Than I put two of the springs on. If someone wants a pic, I can try to put one up, but Associated has a good stock setup.
The reason why I setup My throttle return springs in this fashion is (again) to apply a little bit of brake without any power running to the servo, and to clean up the mounting (as this works on all nitro platforms that I have encountered), and it also extends the life of the return spring, because after time, a throttle return spring doesn't quite work as well as when it was new.