ive had good luck thus far. I just checked the rockers on my new motor, and everything is still tight. I think its still a good idea to check them every so often, just to be certain they are tight, and no potential damage can occur.
I don't think they should ever come off on their own, if you set the lock correctly. Now, I've had a rocker stud break... but that's a whole different issue (well, kinda...)