I would purchase them before semester begins. They tend to sell out in the first few days, and you may be disadvantaged if you have assignments or work due in week 3 for instance. I know a lot of people will purchase on their first day of that class, but if 800 other people were there before you, you may have to wait a few weeks before you get your book!
Last semester one textbook for my class sold out at paramatta before week 1 began, and they only got Luke 30 books in stock so some people missed out twice before they could get them!