Consider this scenario:
my client owes me, say $8000 and sends me one bitcoin worth $8000. So far so good, right?
But I have a bag of coins I bought at cheap prices.
And if I want to cash out the 1 BTC that the client sent, by the rules of FIFO, I can't. I have to sell the oldest coin in my bag.
See the problem?
Maybe you should incorporate a company for this. It's fairly easy and cheap. That way, you should be able to separate your personal Bitcoin investments from your company's since receivables from your client become company assets, not your personal assets.
The tax treatments discussed earlier would obviously change, but it sounds like it might be worth it for you.
One choice is to operate as a sole proprietor, using a single exchange account and keeping track of which bitcoins sold in the tax year came from personal capital assets versus which bitcoins sold in the tax year were those received as payments. If using tax software that automatically imports transactions from your exchange account, then it would be necessary to manually add transactions to the log, because the exchange has no record of the buy price (basis) for the bitcoins you received from the clients.
The other choice is to establish a separate exchange account in the name of an LLC or corporation to handle payments, thus keeping business income segregated from the personal trading of "capital assets," in IRS lingo.
A sole proprietor isn't required to establish separate personal and business accounts, but in this case it might be worth it to avoid bookkeeping hassles. And if god forbid one gets audited, to avoid having to face a skeptical and ill-informed auditor to explain why the transactions that you reported on your tax return don't match the records in your exchange account.
~sigh