I'd say theres a difference between learning 'big words' for the sake of it, and using formal or sophisticated language APPROPRIATELY as this can easily score you marks, and is almost a necessity to get the 9s and 10s I'd say unless you're naturally quite good at English. The main words I'd look out for and try to learn and use are the ones contextually appropriate to your texts which can summarise a longer phrase in a single word. For example, for Hard Times I continually used the phrase 'Utilitarian zeitgeist' rather than 'the Utilitarian philosophy which was prevalent around that era'. Saves you words, and shows off a lot more. Just don't become one of those people who spam long words and lose fluency as a result, and especially don't use them inappropriately.