You don't tax currency, you tax transactions. And you tax transactions based on the value of the transaction. So if I sell a widget for either $10 or 0.01 BTC, and the tax rate is 10% of the value of the transaction, then I'll have to pay $1 in tax for either of the transactions. If I sell a widget for 0.01 BTC and no other amount, then we can establish the value of the widget from other sources, for instance, someone else might sell the widget for around $10 (implying that it the tax is around $1) or someone else might exchange 1 BTC for $2000 (implying a the tax is around $2). Naturally, if you put the decision of the tax into the hands of the government, they'll take the best rate for themselves. You can avoid this by pricing things in terms of the tax rate i.e. using the government's currency.
