It depends upon the country which you resides, in your country if government put tax for bitcoin then you have to pay tax it is necessary. I think most of the country couldn't put the tax for bitcoin. Few countries now put tax for bitcoin, so its purely depends upon the government of the country.
Every government wants to sell money for free. So in the future, they can create some system and we all will be must pay taxes for our Bitcoin. But I will still believe that it will be not created.