I found it helped to diagram when I was first learning and over time I was able to do most of the logic in my head. I'm sure it varies from person to person, some people may write everything down. I think starting with improving accuracy is great and then move to improving speed. So if it helps you, use it! They give you several pieces of paper so it will be there if you need it. (I'm also curious how much others use the paper- when I took the test officially and handed in my materials, the proctor seemed surprised I only wrote on one sheet)