Note: I'm writing this from the perspective of someone who reads a lot of fanfiction, occasionally writes original fiction of a generally non-graphic but weird and at times potentially disturbing nature, and has no triggers but a generous handful of squicks.
I'm generally a believer in authors letting readers have some idea of what they're in for, although if you're writing a mystery story or certain types of mind screw, you'd probably want to be vaguer than if you're writing the warnings for smut. It's sort of an artistic social contract.
I believe that enabling people to avoid things that, regardless of execution, would be automatically bad experiences for them is advantageous for both artists and audiences. One the producer level, you can stop worrying as much about audience reactions to your topic because the people who would find it inherently upsetting now have the ability to avoid it before they've invested any time or emotional energy, and the people who like what you're trying to do will be better able to seek it out. It's kind of like baking - I want people to eat my almond cookies, but preferably the people who *like* almonds, not the people who are allergic to nuts.
The practicalities of warnings seem to pose a bit of a dilemma, though. I don't write much that's graphically violent or sexual, but possibly triggery stuff that's either implied or is mentioned as having happened in the past or off screen is fairly common. Do I warn for Ekaterina talking about how horrible Colonel Shostakov's interrogation techniques are? Dani's stepdad going to Casa Mayakovsky-Lovelace to retrieve her and acting creepy until Kate pulls a paring knife on him? Ada (as a thirty-two year old) telling Chava about the screwed-up family dynamics of her adolescence? Any of the Tykebombs reminiscing about their childhoods? Not that I mind getting detailed if it would really help potential readers' peace of mind, I just don't want to put effort into a huge eye-glaze-inducing header if it isn't giving people information they specifically care to know before reading the story.
(Of course, in my case the whole matter is largely theoretical, as I don't write much, don't post much of what I write, and don't promote it beyond my own journal, but if I ever have a fit of industriousness followed by a fit of self-promotion, this may be an issue.)