If you have the pretest series, there's no harm in doing them. They are a good way to reinforce the material if you're looking for questions. However, keep in mind that the questions are not formatted like the real exam, so don't think of it as a question source but more of a way to reinforce the material. If you don't have the books already, I would recommend you don't buy them. Instead, get a Qbank like USMLERx or USMLEConsult (assuming you're saving UWorld for closer to your exam). If you really want paper sources, try Kaplan Qbook, FA Q&A, FA Cases, Robbins Review of Path, etc.
