The "best" speeds for autorotation exactly mirror the best speeds for powered flight. For best glide, the idea is the same as best range - least energy expended for most ground covered, and the speed is about the same. For least rate of descent, the idea is least energy expended to stay up, so the speed is very similar to the best rate of climb speed. For helos with appreciable speed range between Vbroc and Vbr, the glide speeds are useful. For helos with very slow cruise speeds 9Bell 47, anyone?) there is not much payoff to change speeds.
Also, like best range flight speed, the best glide speed increases with a head wind (by something like the wind speed) and
For the S76, the best glide in auto is about 115 knots, close to Vbr of 125. For least ROD, the auto speed is 74 knots, almost identical to the Vbroc of 65 to 75.
BTW the "best" speeds have nothing to do with blade inertia, because the speeds are used in steady state conditions. Inertia is your best friend at auto entry and near touchdown. On entry the stored energy in the inertia buys you time to drop collective. On touchdown, the inertia gives you a longer collective pull to find the ground.