I fly the Boeing 737NG and therefore can only comment on this aircraft type.
It depends on which mode you are using for your climb/descent;
1. Using LVL CHG the aircraft will fly IAS up to FL260 and the speed will changeover to Mach number and continue to fly the mach number you hae on changeover.
2. Using VNAV it will fly the IAS until it reaches the Mach number assigned in the FMC, this is not dependant on altitude.