Obviously it's not best to rely completely on a formula sheet but making summaries of each topic is a good idea. If you're unsure about a particular section or have forgotten how to approach a type of question, you can easily re-familiarise yourself with the content required. I have summaries for maths 2 unit and 3 unit, both of which have proved very effective, especially when revising for exams and you can make sure you know all the basic theory.