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.
Thanks for the info, i'm in the UK but i suspect the laws will be similar. If it only brings in a few dollars then it would be quite annoying as one would have to fill in all those documents for earning the price of a hotdog.