Where did you get that idea from? Are you implying that if you're running a business and only accept bitcoin payments you're magically tax-exempt until you 'exchange' (which may never happen ie when your landlord/local store/suppliers also accept BTC)?
That's simply wrong. It's not even limited to bitcoin, if you conduct business activity, you need to pay tax even if you only perform barter transactions, otherwise it would create easy to abuse loophole.
I don't know whether there's any official guidance on how to determine market value of bitcoin/other goods or services, but I'm guessing you'd probably be safe to use rate of the major exchange in your country or the average rate of few major exchanges. And if you were to massively understate the value of your income (i.e. by using the rate from some obscure website which offers to buy 1 btc at £1) then you'd be setting yourself up for a tax evasion charges.
Many countries don't recognize bitcoin and have no laws regarding it. I'm sure you know that when you run a business you produce invoices, which serve as a proof of transaction for the client and the government (if needed). Do you make invoices when you mine bitcoins? I think not. When a business owner decides to pay tax and he has no proof of transaction, he has to declare the amount he assumes is right, and I'll repeat assumes, because it's difficult to be accurate on such unstable grounds.
Take a moment and think about it. If we consider mining income what is his actual gain based on?
1. The price of bitcoin at the moment when it was mined?
2. The price at the time of filling up the income report?
3. Or, maybe at the time when he came to his farm and found out how much he has?
4. Or if he mines in a pool at the time when the coins appeared in his personal wallet?
5. Based on what exchange and what currency? In some countries there are no exchanges until this day, while in other you have to convert local currency to USD to perform a transaction. What about countries in which there's a discrepancy between the price in local currency and USD or the local price and the average.
Again you said about business activity. Of course if you are allowed to register a business dealing in bitcoin you will be required to fill out a form each year, but that wasn't OP's question.
As long as we keep Bicoins as btc in our account and don't exchange them to either EUR or USD will we be liable for tax at this point?
And my answer is still no. Until he exchanges it for fiat, goods or services, he doesn't have to report and pay anything.
1. There's nothing that links him as a person to his wallet account.
2. The value of the coins is so unstable, that one could say the sum of his yearly income becomes larger each month.
Last question, just for fun. If you were given an item of value, like an antique painting, and you knew you're not going to sell it so its value wouldn't matter to you at the time.
Would you go through all this trouble to find someone to price it, pay that person, fill out the tax form, and run to your tax office to report it?