It all depends on who you are writing to. If you are writing on your blog to a group of academics, of course you need to be accurate with your spellings and grammatical structures. If you are writing to your friends, a mistake here and there won't likely hinder their ability to understand your blogs. The same goes with writing an essay. Though you usually write an essay in the context of academia. But if you write an essay just to express your opinion in the school newspapers, you don't need to be overly formal. You can use slang and idioms more frequently.