Here is what the B407 RFM has to say:
18. Rotor brake (if installed) — Apply full
rotor brake at or below 40% NR.
Return rotor brake handle to stowed
position just prior to main rotor stopping.
<emphasis added>
I know that the thread asked about the B206 but as others have introduced other types, I thought I might as well. On the 407, if one leaves the RB fully applied until the rotor stops, it stops very abruptly, possibly putting inappropriate loads on the drive system. I have been advised to apply the brake at 20% unless there is a reason to do it sooner. This allows Ng to spool down so that by the time the rotor is stopped Ng is at 0%, allowing the battery switch to be turned off with out FADEC piston parking problems. Bell specifically warns against stopping the composite blades over the exhaust stack, so I let the blades slow almost to a stop with the brake, then release it and gently tweak the blades to a stop at 45 deg angle to the fuselage/exhaust stack with the RB. Seems to be within the RFM recommendations and works very nicely.