Like so many things in helicopter aviation (e.g. hot refueling, getting out with the engine running, scud-running), using the collective and/or pedals to slow the rotor after shutdown should be accompanied by the caveat: DON'T BE AN IDIOT! USE SOME COMMON SENSE!
The Bell 47 indeed once allowed for up to 1/3 of the collective travel to be used for slowing the rotor. Even if a 47 had a rotor brake, it would barely stop a Schwinn bicycle much less a set of high-inertia metal blades. Once the 206 was invented (and with it a better rotor brake) that procedure disappeared from the flight manual.
Then later on Bell got all antsy about people using left pedal to slow the rotors, because the tail rotor could bang against the flap stops. But banging against the flap stops can happen even when shutting down in strong crosswinds and stuff. So the wise, old 206 pilot trims the pedals to minimize this during shutdown. Sometimes it takes left pedal; sometimes it takes right. You have to find that "sweet spot."
If you've flown a Hughes 500 you know that there's a spring that pulls the left pedal forward as the rotor slows. Sort of an "automatic" rotor brake.
In my non-rotor brake-equipped Bell 206B, I sometimes use a bit of collective and pedal to slow the blades, depending on how badly I have to pee. I said a "bit" of collective. The forces are certainly no stronger than those imparted by a hovering auto (or powered flight in the case of the t/r), and I'm careful to not let either of the rotors get flapping too wildly. I do not just yank full-up on the collective and jab full left pedal in and then exit the a/c in a big hurry no matter how urgently I have to do the aforementioned deed.
Use the brain God gave you, and don't be an idiot. Apply a bit of common sense and be smart about these things. If we all did that, perhaps we wouldn't have flight manuals that are as thick as the Manhattan Yellow Pages, with all sorts of admonishments to the stupidest among us.