If you convert your bitcoin to the fiat and then you should pay the tax since you are literally making profit by convert your bitcoin to fiat, however it's depend on which country you live due to rules and regulation of each country is different. Unless you don't need to pay tax when you hold your bitcoin on your wallet or transaction with them.
yes there is no tax on bitcoin directly but if you change your bitcoin in to fiat currency and deposit it in bank they you have to pay tax, actually bitcoin is not yet consider as legal in any country therefore there is no tax on bitcoin any where.
yes that is right to say, as not a single country has yest consider bitcoin as legal currency therefore they are not putting tax on bitcoin but once it become a legal currency then they will certainly put tax on bitcoin, may be some countries are thinking about this to legalize it, and if once it become a legal currency then it will become more and more popular currency of the world.