There is no such requirement. Some writers and editors prefer to just leave the word plain, some write it as NOT, some choose to make it stand out as *not*. It doesn't even have to be spelled out - a contraction such as isn't is also acceptable. I personally make it NOT because I got tired of handling correction notes from players who hadn't seen the word as they raced through the question, but it's a personal choice. As an editor, I encourage authors to make their negatives stand out for the same reason.
(Editor in Humanities, Religion, Literature and Books For Children)
That's all, folks!