There is a common misconception that because of the way pedals are threaded, that they are self-tightening. While this is true, in practice, I have seen hundreds of crank arms stripped out because the pedals were never tightened with the proper amount of torque during assembly. If a pedal is not tightened down enough, it rocks in the threads, chewing up the softer aluminum of the crank arm, until it eventually it falls off.