If your school has an integrated curriculum (classes structured as cardiovascular, pulmonary, GI) instead of a traditional curriculum (classes structured as anatomy, physiology, microbiology), it may be suitable to start studying in the summer.
Reasons to review:
1) Each time you review, you remember more, so the more times you review, the more you remember at the end. In the integrated curriculum, you've already learned both the normal and abnormal processes for at least half of the systems, so there are plenty of high-yield topics that you can review.
2) Students in the traditional curriculum learn a lot of concepts twice (esp the ones related to physiology), the first time during the first year when learning about normal processes and another time during the second year when learning abnormal process. Students in the integrated curriculum don't have this "built in review" so it may be good to review in the summer.
Reasons not to review:
1) Takes time. Who wouldn't rather just relax by the pool or travel abroad instead?