You usually have to pay taxes on coins that were converted to fiat. As far as the tax on held coins go, you don't have to because it's unexecutable by the government, since addresses are not linked to identities, and I doubt this will ever change. They may say that you should pay, but they still won't be able to track you, as it would cost them too much. It's much easier for them to track the flow of fiat money.