Great thread...
I agree with mjs and his analysis...it is true, it requires guts to go the extra mile on the highway of horror, and to take certain exits no one else dares to take. As stated, the constant reader is left with her own subjective analysis.
When you say does he go too far, for me, I take that more to mean when has it personally made me too frightened, not necessarily that society disapproves (although society does do that at times, for sure). King has done that for me; there are certain scenes I won't read again. There are many in Rose Madder that I just can't read again, yet let me say this - those scenes I am thinking of were incredibly well-written. They show just how much of a genius King is. I have to say, too, in all seriousness...don't ever think King isn't a genius. He is...there is something about his writing that is just superior all around: technique, execution, originality, generation of atmosphere, construction of symmetry, innovative use of words/grammar/dialect/punctuation...he can take your worst nightmare and market it back to you such that you will buy it with emotional currency you probably didn't know you don't have. (Besides Madder, some parts of The Stand are tough; Flagg is not a funny character, and that universe also has non-Flagg scenes that are pretty scary, for me, anyway.) I'll also point out King is the master of the metaphor, especially in his earlier stories; it is so difficult to constantly come up with clever, relevant, poetic metaphors on a constant basis within the prose narrative. Try it; it's work.
Someone asked: is King getting more frightening/crude in later writing? I agree, he has, I think. I agree too with the sentiment about some earlier stuff...more EC than anything else. (Someone mentioned Bentley Little: I'll tell you, I love him, he is afraid of nothing, but there was one thing in his latest collection of short stories that even I had to pause over; still, that particular story is awesome.)
I never would want to restrict King, though. I think all writers should be given that freedom. I myself, mostly a hobbyist, have already chickened out a couple times with something I am writing on Wattpad. King would not have done that, even when he was starting out. Wish I had that literary courage...