Here my idea: All the btc (and alt coin) exchanges should agree to have no rate between any fiat/coin and coin/coin on the day(s) that counts for paying taxes. I'm very sure it wouldn't be that easy as it sounds and there are different rules and regulations in different countries for that behavior of course but I just want to put the idea in here. Maybe it's an idea for another altcoin? Let's talk about that.
But that's not what you were asking was it?
Bitcoin is not unique as a commodity with a variable value in relation to the fiat with which you run your business so I don't think you need go very far with trying to get 'agreement' here on a usd/btc rate (you'd be pissing in the wind with that one anyway). If you're going to be straight up with the taxman as to at what exchange rate you received and at what rate you converted to fiat there's no reason you should need to pay 'too much' in taxes either. If you made on the transaction that adds to your business profit and if you consider the whole of the business profit as taxable then you'll pay only what is due.
I have seen threads on this forum on ways to deal with Bitcoin profits (from mining or speculation) for the purposes of US taxes and I expect there are others for other jurisdictions too. It may need a bit of getting to grips with (or for your accountant to get to grips with) but I can guarantee you it will be easier than trying to get folks here to agree to change something about Bitcoin itself or about interfering with the free floating, market-driven exchange rates to suit your tax return!