Sierra5913, normal Qantas policy is econ speed at a given (fixed) FMC cost index which varies with aircraft type, but there are caveats which involve staying close to schedule too. So the answer is "it depends".
Also, Qantas has become so dysfunctional in the Dixon/Joyce era that the left hand doesn't even know the right hand exists. No one department cares what any course of action they take is going to cost any other department. They only care what the cost is to their department directly. If one business segment implements a policy which saves them a million dollars on paper, but costs another business segment two million dollars in flow-on effects in the real world, that's fine.
So the pilots do make decisions to vary the speed based on many factors, like "is a late arrival going to cause major disruption to the next crew/aircraft schedule" and so on. If we didn't take these factors into account and just let Qantas run itself, the wheels would fall off rapidly.