Honest opinion: NOPE if you’re already aware of what NIFT entrance exam is about. But even if you’re aware, there’s no harm in joining one (if time & money permits) since coaching classes gives you a habit of practising regularly and improves your pace. It doesn’t matter if you practice at home or at coaching classes. More you practice, the faster you solve the questions. and trust me. TIME LIMIT IS A VERY IMPORTANT FACTOR IN NIFT EXAM.
Another plus point joining coaching classes is that you always have some one to help you out when you have this ‘creative-mind-block’ phase or have other doubts.
What you have to figure out is if it is all necessary at the whopping cost most coaching institutions charge.