@kikir i'd say when in doubt, use it. it can be a warning, a topic indicator, a header for a lengthy post, pretty much whatever you want it to be
some personal examples - i tag mental health because sometimes i talk about things that others might not be equipped to deal with right at that moment. i tag sports because i know there are a lot of people on here who just don't care and would rather scroll past anything with that header. both of those uses are valid and important imo
i think the key is really just try to use your best judgment on what might be tough for others to see unexpectedly (and of course if someone explicitly asks for a CW, use it)
@ebeth thanks for sharing your thoughts and examples!