I agree with Mariana. In the formal way of writing, we never write it in katakana, however, in the casual way, for example, text messages between friends, we sometimes use katakana. In the casual way, there isn't specific rules, we can use whatever we like. So, I think it might confuse the learners. The same happens in English and other languages, doesn't it? Like "luv ya", "2 you", "4 you", etc...