I would recommend going early to get the
used books that are in good condition.
Used books do go fast, and cost much less than the new ones. They are usually the same edition, so people want to go early and pick up all
used books to keep costs low. If you wait until classes start, all the
used books may be gone.
Remember that if you buy
used books and it turns out that the professor tells you that you don't need that book anymore, you can always return the textbook and get a refund. You just can't refund custom courseware (so always buy custom courseware last). Also, you can most likely wait till classes start to see if the 'optional' material is really necessary.
However, if you don't care about buying your textbooks
used or not, then feel free to wait till classes start, get the finalized list off the prof, and buy your
books later. For the years I've been at mac, the list on the Titles website hasn't differed from what the prof has told me to buy.
So the choice is yours