If you are flying a 4 stroke and using metal gear servos, you better use something to lock it down or they will come out, arm and all. I had it happen on two servos in one day and was lucky I had two elevator servos and the other was rudder. You don't need it on nylon gear servos, but with metal you really do. The screw will work it's way out and I checked them all at the beginning of the day. I was into my 4th flight when it did something stupid on a loop and decided to land. Didn't know I lost rudder until I tried to turn it on landing.
Ed