I think it's a mistake to try to give a "one size fits all" answer regarding exact temperatures because different types are affected differently.
Torque increases can be a good indication that ice is building on the main rotor system. Irrespective of ice build up on the blades themselves, build up on the rotor control system such as pitch rod ends or servo valve links might be more critical (the latter is a check to be carried out on the S-76 before flight in low temperatures).