It's not going to hurt you to read the LGB and LRB beforehand. Read them as many times as you like. The more dangerous proposition is taking practice exams. The concern is that you may develop bad habits, and you don't want to blow through the universe of actual LSAT questions too quickly either.
If you're still chomping at the bit after you get through the LGB and the LRB twice, reading carefully, and you still have plenty of time before your class, then taking a few exams before you start the class isn't going to irreparably harm you, most likely will help you, and should leave you with plenty of material to work through closer to exam time.
As with any self-study program, you want to start off by taking an exam or two without keeping track of time, focusing instead on applying LGB and LRB concepts to the questions. Once you have taken a couple untimed exams, you should start keeping track of time with your silent timer, while still taking as much time as you need to answer each question. After having taken 4 or 5 exams in total, note which types of questions you struggle answering both correctly and quickly. Return to the LGB and LRB to focus on those question types and to relearn and master the appropriate techniques to address them. Return to the tests, taking them under timed, test-like conditions. With every test you take, make sure to go through every answer/question and think about both why you got right those you got right as well as why you got the others wrong.
Feel free to contact Admit Experts (firstname.lastname@example.org
) for more information about designing a self-study program.