Currently, you don't have to pay tax on bitcoins. However, as long as the number of people using bitcoin keep rising, i think the government will develop a system which require users to pay money to use bitcoin. Well, the government can do anything that they want. Trust me
but i think the government cannot develop a system to impost tax on bitcoin until they legalize it, and still i do not think that any country has officially legalize bitcoin, i think still any government given any statement about bitcoin that either it is a legal currency or not, and therefore still there is no tax on bitcoin. may be in future when government when the government will legalize bitcoin will think about to impose tax on bitcoin.