They do need work to do! It doesn't have to be sheep or even something formal, but they need to be a part of your life, doing stuff like fetching papers and playing games. They tend to develop rituals and decide certain things have specific rules. Its either cute to you, or super annoying. I usually find it cute. But I am a weirdo.
But really, Aussies need brain work, too. Any active working breed does.
True that! I believe ALL dogs are "working dogs" and need a job and job description, even if that job is to mostly lie at your feet and be your constant companion. That job has rules, parameters and expectations, so work with your dog and be sure he understands and complies with them. A responsible dog is a happy dog!