Well, based on my experience at my school, knowing what the professor wants has more to do with how they want you to format your answer--IRAC, flowing IRAC with MOR analysis (more essay style), lots of case names and explanations, or doesn't care whether you use cases. In my experience profs don't mind telling you which format they want. It helps them to tell you because they will have to grade 70+ exams, and would rather grade one structured the way they prefer. It doesn't seem to impact how much you need to know the law. Regardless of which one they want, you need to know the law really well, and you have to know how to apply it.