First of all, check whether Bitcoin is classified as a currency in your country or not. In 99% of the cases, it will be classified as an "asset" by the government. In such cases, the bitcoins that you receive from signature campaigns are not taxable (as long as these coins remain in your wallet).

However, the moment you convert them to fiat, the revenues from that sale become taxable. And the worst part is that whatever revenue that you receive will be 100% taxable as regular income. You can't show this amount as capital gains, to qualify for the lower tax rates.

This is my understanding of the situation. If someone disagrees with me, then please feel free to do so.