No, you do not have to use interfacing. (I used it on one bag, but not the other.) It really depends on the use of the bag. As an example, I love adding interfacing to bags with boxed corners as the interfacing helps the bag stand up. (Which can be handy as a cosmetics bag, imagine the bag on your counter and how convenient it is to just toss in what you need. But if making a bag to be used as a pencil pouch. I personally don't think interfacing is really needed.