I wouldn't bother. The amount you can earn from these campaigns are quite small, even for Legendary ranks.
That's right. If to cash out by little like 50-100$ a month a bank won't has any pretensions to those money. Taxation is more about big sum of money which source you should represent to the bank and taxation system.
It doesn't matter how much you cash out, you're being paid in something with value and that's the place you're taxed.
Of course if you don't bother reporting it in most places no one's going to find out, but you might be setting yourself up for future problems if they choose to sniff around. Again, the likelihood is low.
And for some campaigns the payouts can easily take you above your tax allowance. I can't be bothered to add it up but it's quite possible I went over mine last year purely from the campaign I'm in. If the price had stayed high it would've been not too far off the average salary where I am.