I just use the default "Mature content" warnings on Tapas, even for stuff that I personally don't consider very sensitive. I think to myself, "Hey, would I be OK with a kid under fourteen (as a general, not precise, marker of innocence) reading this and then talking to me about it afterwards?" If the answer is "uh, no," then I use the content warning.
As an example I've done this once with the chapters of my novel so far. The 'mature content' in question is the obvious implication that two characters had sex, some rather direct innuendo, and the description of nudity.
It's nothing offensive and doesn't involve any unsettling themes like non-consent, but I just wouldn't want a kid to read it, because there's a bit more romance going on than a smooch.
I've had a sword-fighting scene in an earlier chapter with death in it, but it's not especially violent-feeling since the focus of the writing is on the character's skill with a blade— not the spray of the blood or the expressions of agony of his opponents as they die. You know what I mean. It's "fantasy violence," 15+ style, not 18+ style. If there are times that I want to depict a fight in a purposefully upsetting way (in an 18+ way, the sort of violence that makes you want to skip the scene in a movie), then I would use the appropriate warnings.
If you have really sensitive/dark themes in your work, like suicide, sexual abuse, domestic abuse, child abuse, explicit sex, gory violence, drug (including alcohol) abuse, cannibalism, slavery, or extremely inappropriate language/slurs... then yeah, maybe you want to give a little heads-up at the start of your story. Or, just foreshadow these flavours of tragedy nice and early on.
For instance: a main theme in Blue Eagle, Cricket Prince is vengeance. The type where you ruin the person you hate and then kill them unpleasantly. Death abounds, too— after all, as my synopsis says, there's a war going on.
Zhisen, the main narrator, makes his rage, his intentions, and his memories of his brother's brutal death by fire (yikes, a burnt corpse) abundantly clear in the prologue. The prologue warns you: "hey, this is not a sunshine-and-rainbows story!" There's death, there's grief, there's violence. But the prologue and synopsis tell you all this.
That's a classy way to go about warning people without spoiling things. Foreshadow well, and if you're really worried that you've created something extraordinarily upsetting/triggering, then maybe add a little note to the reader at the start of the book to say, "Hey, if you've got trauma, this book might not be for you!" Me, I know that my book doesn't have stuff like sex scenes, persistent abuse, drugs, nasty language, slavery, and such in it. I know that people who read historical fiction and fantasy novels aren't going to be shocked by the kind of mature content my book has in it. So, I just do my best to make sure that the tone of the first chapters reveals the nature of the story.
I hope this helps a little!
