TN applicants purchasing plane tickets

I have a question to ask if I really need to buy a plane ticket before my interview? Do I need to have it two way or one way? Should I really needs it in advance? Is the border officer will ask for it? I am still going for an interview this July in Dorval airport (Montreal) and I don't know what to do.
Anyone (TN applicants) who has the same experience in this airport. thanks a lot

Why would you need a return flight? If your accepted for the TN, you have the right to stay in the USA for 1 to 3 years. Return flights are for people coming to visit, and it shows they are leaving. Not for TN's.

TN visa have multiple entries, does it mean I could travel back and forth? I am planning to stay there for a week and come back Canada to wait for my SSS or do I have to wait only in US. Thanks for the reply

TN-1 was expressly designed for cross border workers. If you reside in Canada and go back and forth regularly then clearly you have non-immigrant intent.