I am not sure if it is a detail. The most intuitive way, in my view, is to say “stop” when you want that your assistant stops what it currently does in a narrow sense. If it talks, it should stop talking. If it displays some specific content, it should stop displaying that specific content. If it plays music, it should stop playing music, etc…
The semantic ambiguity of “stop” may very well require that Mycroft understands much more context - and ultimately also personal context (that can become personal data if Mycroft collects enough to finger print users). So, I think that skill etiquette depends on/interacts with data privacy at least.
Probably, it also depends on accountability, e.g. when an assistant does something for you that you rely on. For instance, you may want Mycroft to order 10 pizzas for your birthday party. While Mycroft utters that it successfully sent the order, you quickly realize you want someting else and tell Mycroft to stop. Does that mean it should stop uttering or cancelling the order? Should there be a different “default” for skills that perform something that may cost you money?