I'm fresh from a Youtube internet argument, and I'll tell you my view of thing: proper spelling and grammar really, really, does matter.
It distinguishes you from the millions of other retards on the internet - if it's something like "nvm ur to ignorant to see the point", well, people can safely disregard it.
I don't use capitalisation in chat and multiplayer-game conversation, and sometimes I don't use it in *chan posts, depending on how mature I intend my post to be, but I think that's it.
I also don't see why internet-bad-grammar would leak out into the real world in your exams and essays...