I started revising for exams before starting lessons, but did my exams during my PPL. The dilemma is that if you know the theory, you get more out of the lessons, but some of the subjects make more sense after you've started flying. I started with the topics I already knew best (human factors, aircraft systems) then worked my way through the more straightforward ones (performance and planning, for me as I'm fairly numerate and remembered some of the physics). I also did air law relatively early, as many schools require this prior to your first solo.
How are you planning to structure your PPL? I did mine intensively, which changed how I approached revision as I tried to get to grips with most subjects before starting. I also used airquiz.com a lot to identify gaps in my knowledge.