Mach indications aren't considered unreliable below 25,000. However, we generally don't go fast enough to be concerned about mach effects until we climb above the "cross-over" point, typically around 27,000 to 29,000'.
At lower altitudes, one will typically bump up against Vmo before reaching Mmo. As one climbs, the mach values will occur at lower and lower indicated airspeeds, and a transition from using indicated airspeed as a reference, to using Mach as a reference, occurs in the area of 28,000' during the climb and descent.
For aircraft that are capable of exceeding mach 1.0 at lower altitudes, mach is still a reliable indication. For most airline, corporate, and general operations, however, coming close to those values at lower altitudes ins't going to happen, and airspeed limits take precedence. Once at higher altitudes, airspeed is no longer the maximum limiting factor, but mach.