Here is my personal opinion, some of which is based on fact. If the POH states that you should not move the main rotor using the tail rotor then you should never use the tail rotor to slow the main rotor. It has to do with torsional windup of the tail rotor drive shaft. The R-22 has this requirement.
There should be minimal flapping of the tail rotor when on the ground and the main rotor is winding down. The flapping and attendant pitch flap coupling (Delta-3) is to equalize the lift across the tail rotor when in forward flight. During the main rotor slowdown, there is minimal if no downwash and no forward speed component. So, when you add left pitch to the tail rotor it will act like a propeller and the loads on the pitch bearings (Centrifugal and thrust) are less than those that would be encountered in flight.
But as indicated above the rotor brake on some helicopters is on the tail rotor drive shaft. In cases like this application of the rotor brake will cause reverse torsional windup of the tail rotor drive shaft and the designers will have calculated these loads taking them into consideration during the design of the tail rotor drive shaft and support bearings.
The pendulum swings both ways in this situation and as pilots it is better to err on the side of conservatism.
