I'm not American, but the thing with taxes is (especially if things are undefined / unclear in the law) that you need to go with the most straight forward solution so the tax authority can't call your "solution" unreasonable.

If you earn $100, you'll pay corporate tax on $100. If you earn 1 ETH on the 1st of January, and the value of 1 ETH at that point is $500, then you should value that ETH at $500 and pay the tax on the corresponding dollar value (in this case $500). You're not going to run into any problems there, as it is pretty much the most reasonable approach.