Most aircraft have a Vne and an Mne - obviously the relationship between the two is a function of height.
So at lower altitudes it's always CAS/IAS that's the big player - at altitude IAS becomes fairly small but Mach becomes much more significant. Since that becomes your operating limit, it makes sense to use it for other purposes as well.
Example (fictional aeroplane) Vne=300kn, Mne=0.7. At ISA S/L conditions 300kn = 0.45M, so you can ignore Mach and fly to IAS limits quite safely.
However at FL240, 300 kn=0.7M, and at FL 300, 300kn=0.85M - well above Mne. So above FL240 you may as well ignore IAS (well apart from for converting to TAS for Navigation anyway) and use Mach all the time.
G