In fact we should be reporting our monthly bitcoin income from signature campaigns, but who the hell does that? I would do it, but im too scared to get a penalty for not doing so earlier, and I also don't know what kind of proof they will demand, I just don't know what to expect, and nobody has clear answers for this, therefore I've resorted to holding it all until I have a clear vision of what we are talking about in this case.
It depends per country. I have been a freelancer for a good while, and haven't had one single time where the tax department required me to provide any proof. I recorded everything obviously, for in case they did ask for it, but it didn't happen. In some cases I had to pay like 50% in tax over all my earnings as freelancer, which is just insane. Don't get me wrong, I don't mind paying tax when reasonable, but this is a clear form of theft, and for that reason will never pay tax over my signature campaign earnings, and I'm sure others think alike -- just don't let greedy governments take away even more from you. Don't wake sleeping dogs....