It depends on your finances. If you have the money to buy the books before your first class I would. This will ensure that you actually have a copy before class starts. If you wait too long, you may not be able to get the book and have to wait. This might catch you in a bad place where you find yourself behind in the class.
If money is an issue, wait. This could provide you a chance to see if there are alternatives you can use other then buying/renting the book. If a book is only used for one chapter, why pay when you can check it out in the library?