I've actually done a couple of hours of instructing, mostly in the R22, some in the B47, H300 and a bit in turbines, B206, B206L, H500 and B407. My opinion is that touch down autos need to be done during initial training (I'm talking civilian, not military, so piston helicopters). Over here some schools do them to death, almost as though it happens that often that you need to be very good at it.
I think limited power is a more likely situation for helicopters than engine failure, it can happen without mechanical problems.
Fact is engines do fail, unfortunately, more often than not because "old mate behind the wheel didn't bring enough juice". So with that in mind, yes you need to be proficient at autos to the ground (or maybe make sure you your engine has the required fuel and maintenance).
So how often should you do touch down autos to remain proficient? (I've got no idea).
The question is how much money do you want to give insurance companies. It won't be long and schools won't be able to insure their aircraft for touch down autos as it will be cost prohibitive. Then what do we do.
I don't have any good answers to the problem, but who has???????????