The principle of defending one's, and the related principle of attacking the opponent's, weak (or more accurately: unsettled or unstable) important stones is not a truth because it has exceptions. However, one can reformulate to make it a truth:

Defend your weak important stones, unless you choose on purpose an at least equally good different strategy.

Examples of different strategies are: sacrifice the stones; defend the stones indirectly (e.g., by attacking nearby weak opposing stones to create a mutual running fight); postpone defense to do something else.

Perhaps we could reformulate this as "You need to either defend your weak stones, or follow a strategy that involves them being attacked or captured."

This strategy could be sacrifice, but it may also be a sort of amashi technique or something different. It also gets away from the need to determine if a stone is important or not in the formulation of the wording, since a strategy that makes stones important or unimportant will determine that. As far a good or equally good strategy, I think it's very difficult for a beginner to tell if a strategy is good or not without experience trying different strategies and failing or succeeding, though there are heuristics that help.

We?

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Patience, grasshopper.