Legally, tax obligations have everything to do with the country you live in. If you get caught not reporting significant income, it could mean going to prison in the worst case. It probably is mandatory to report any and all income anywhere but you might get those taxes you have already paid deduced from the taxes you have to pay where you life.

I'm still not sure what taxes I should pay from Steem income and when. I earned very little last year and none in fiat. Now the guidelines have changed.