Yes it uses the average price for the day (worst case).  If the reward is in the past few months, the BTC price is gathered every minute (it gets averaged out to 5 minutes after a couple days old).  In most cases it will be the pretty much the price of BTC when the block is found.  I may offer different ways to calculate this in the future (when it gets to your wallet, etc), but this is one of the ways that the IRS recommends you report this if you want to write off your power, equipment and other costs.  So what minergain reports is your income and you already have your costs I suspect.  Then when you convert your BTC to your bank, take the difference of your final bank dollars minus what you reported and that is your long term gain (or short if less than 6 months).  Obviously this is still the wild west as far as "if" and "how" you report, but this will make it much easier if you decide to.

Also, I have daily dollar to about a dozen other international currencies for the past year, so if anyone needs that, let me know and I will get that implemented sooner than later.  You can see the list on this page: https://www.minergain.com/all_virtual_prices