I wouldn't bother. The amount you can earn from these campaigns are quite small, even for Legendary ranks.
I don't think no one will harass you for such a unusual income that no one is sure about.
It may be a small amount in Spain, but here in India the tax authorities will issue a notice even if you fail to pay $10 in taxes. It is quite irritating and time-wasting, but this is how the tax authorities work in the third world nations. That said, I am not going to pay any tax on my signature earnings, until I convert my coins to fiat.