I am responsible for my own spoilage. If I choose to read lj before I finish the book, and cannot resist poking at cut-tags, that's my problem, or if I read a cut-tag that is more informative than it should be - also my problem.
I trust that people will try to put things under uninformative cut-tags, but I know that mistakes happen.
I will also do the same for my own lj - all essays, stories, filks and rants referring to HPB will be cut-tagged for the next three months. Which is what I normally do anyway, but for that time period, I'll say "spoiler warning". Also be aware that a post that is, in itself, spoiler-free might have spoilers in the comments.
This is where Shabbat becomes really, really *good*. I won't get a chance to read LJ until Saturday night anyway, by which time I'll be deep in the book.
(On another note: eating a large burger at 11PM is a *very* bad thing for my tummy.)