It depends upon the tax laws in your country. Some places require you to declare every penny you earn, while some countries do not apply income tax below a certain threshold.
In Canada, all business income must be reported, and the CRA (our tax people) determines business income as: A business is an activity that you intend to carry on for profit and there is evidence to support that intention.
Obviously, commission junction or any ad revenue would be done for profit, so must be declared.