The Mach to IAS conversion is fixed for each pressure altitude (ie Flight Level), thus the Mach number is said to be independent of temperature. The operator specifies a climb & descent speed schedule, so one operator might say "Climb at 280 knots IAS into Mach .76", while another might say "Climb at 300knots into Mach .78" - so the Flight Level at which you change is determined by the operator's SOPs.
In terms of when Mach number is displayed (IAS is always displayed) - this depends on the instrument type & fit.