So in this site's story pages they already include content warnings you can mark off. On some, people though state that particular chapters have 18+ stuff in them while others don't. Is it better to leave your chapters blank concerning this and say, "I already noted this would exist in my novel by checking off the warning labels," or is it better to absolutely do so?
It's entirely optional. I do simply because it's a minor issue for me but can make a difference to my readers, plus my chapters marked NSFW tend to get more views than those that aren't.
Additionally, I have found that by having individual chapters marked as R-18+/NSFW, I seem to get more readers. If a story is marked as smut, a reader may well assume that the entire story is going to just be sex after sex after sex. By denoting certain chapters have sex scenes, it lets those that don't want porn that the story itself is plot that just happens to contain sex, rather than fantasy wish fulfillment. sadly, there's not clear delineation between plot with sex versus porn-driven in the world of writing. I've heard that erotica is plot with sex and smut is pure sex, I've heard the opposite, I've heard that any written story with even a single graphic scene is porn in it's entirety... point is, there's no clear boundaries or lines. So by marking my chapters, I let the readers know that it's plot driven and just happens to contain sex.
Sorry if I rambled a bit. I've actually done a fair amount of research into this.