When to train new models

When using Rasa X, I can add stories, change rules, add NLU, etc.

Since I’m doing all these changes…when it is appropriate for me to train a new model? Is it only when I add new entities and annotate entities that I need to train a new model? I know when I change pipelines, that’s a time to train a new model because I’m using new ML. But what about when I change rules, stories, or simple NLU? Should you always train a new model there too?