Depends entirely on the unit you're doing. I had an Arts unit last semester where it would have been simply impossible to have passed the exam without the textbook, because the questions were based entirely off it. Similarly, I had units last year where the books acted as expensive paperweights - I opened them once or twice but that was it.
My advice is wait until closer to the start of semester. Every unit has a "unit guide", which lists information about the unit. This is generally released the week before O-Week. For all of my units last year, this included the set readings for each week. You can get a very good idea of how important the textbook will be by seeing how many weeks you need to read from it. If it's just a few, I wouldn't bother - you can always borrow the library copy for those. If you see it being referenced week after week after week, I'd try and get it if possible.