i like warnings - I probably don't really want to discuss this
I'm trying not to read the current warnings thing in fandom, mostly, because I think reading posts about that would probably be a great way to make me hate people. But!

(a) Yes, I actually don't read published fiction any more pretty much ever, unless it is comics (though I don't know that Marvel uses the rating system any more?). I mostly read non-fiction which is not so likely to have surpraise! self-mutilation in it. Though European history always has some pretty nasty stuff. Though I will say a relatively recent issue of the comic X-Factor contained a plot element that is a sensitive issue for a lot of people, caused a lot of people to drop the book and other people to start reading it, caused a lot of discussion, wasn't warned for as it would spoil the story. Which was interesting.

(b) TV and movies actually do have warnings. Um, in case people arguing that they don't hadn't noticed the warnings before TV programs and movies and on movie posters. For example, Australian TV (at least I think all the free to air stations do now, and not just SBS and ABC) warns aboriginal viewers when the following program will contain images of dead people, which I think is very considerate of the audience. Sex scenes, violence of varying degrees, sexual violence, supernatural themes - all warned for on TV, at least in Australia (I can't speak for other countries obviously).

(c) When written by crap authors, stories with self-mutilation actually just make me roll my eyes and laugh, before mocking them on AIM. It's only when written by competent writers that SI in fic makes my hands shake. So, if your writing sucks, be comforted to know you are not the reason I have to keep all the knives in the house hidden.

Edited to add - I actually find adding warnings fun, even for silly stuff that doesn't really need to be warned to, like when I went to town with the warnings on this fic. (Warning: Don't read unless you like fanfiction that sucks.)