1. At each level in the hierarchy, there should be an unambiguous distinction between the values. This means that there should not be any overlaps in definition (or in the values collected at lower levels of the taxonomy), nor should there be any gaps (i.e., missing values within the level).
2. There must be a coordinated approach to modifying the taxonomy. In other words, when it is clear that there may be gaps in the value set or that there are values that imply the introduction of new levels in the hierarchy, there must be a “political” framework in which the new elements are proposed, debated, modified and approved to maintain synchrony.
There is a “political” framework involved even when you are defining a taxonomy for your own, personal use.