Most systems of classification or quantitative measurement depend on standards. It is of the greatest importance for the user to be aware of the mandate of a particular system. If the mandate is not understood by casual users, a system can be either underutilized or abused.
In the particular case of the MK system of spectral classification, types are defined by the standard stars. They can be calibrated, and the calibration may evolve with time, but the types are relatively stable because they are defined by the standards. The autonomy of this powerful system is crucial to its success, but some astronomers do not understand the importance of this distinction. Recent suggestions to change the spectral type of the Sun show an ignorance of the way the system works.
Precautions in the use of standard stars and the frequency of their use depend on the particular system and on its mandate.