So, even in a perfect world where BTC is the only currency, don't you think there should be a way to let everyone pay their taxes? Then HOW, that's the tricky part! Directly - you pay depending on what you earn - this could be done by your employer OR indirectly -you pay when you consume - like VAT? Dunno... 
Why do taxes have to depend on what you earn? Why not just have a simple system where everybody owes the same amount of money every year, wouldn't that be more fair?
The how of paying taxes with bitcoins is really easy: the government sends each citizen a bitcoin payment address, they send the bitcoins owed to that address.
Yes, they must depend on what you earn.. How can you ask a citizen earning 10 to pay 5 and also ask another citizen who earns 5.5 to pay the same amount. What about a person unemployed? How can he pay 5? That's the principle of progressive taxation, the one earning 10 will pay more than the one earning 5, so that the unemployed can still receive some services even if he cannot pay taxes.