With BTC there isn't a broker so I assume you simply need to document your decision but I'm not a tax lawyer so I'm not really sure.
The blockchain is the most perfect broker record you could hope for.
With coin control you could sell exactly the coins you wanted to at exact the basis you wanted to at any time.
D&T: what do you think about the legal implications of my zero-gain/loss-for-tax-purposes wallet idea. It uses coin control to select a suitable linear combination of coins such that each and every day-to-day purchase you make has a gain/loss for tax purposes of identically zero.