Actually, you'd declare the initial gain this year (for 2017 results), and any losses due to price fluctuations would be declared next year (for 2018 results). You can disagree, but you'll be putting yourself at risk of an audit, and potential fines and increased taxes, if your holdings can be identified by the IRS. Especially if you ever report crypto holdings, or exchanges report that you have crypto holdings.
Realizing the gain in a Fiat currency is not actually required. Holding a cryptocurrency is treated like property in the US. When you convert that crypto into a different crypto, or Fiat, you will realize a gain or loss from your initial investment upon conversion. That's the law as I understand it. I don't have to agree with it, but that's what it is. If you choose to not report in this manner, that's your own choice.
This is not financial or tax advice, I'm just sharing my understanding of US tax law as of the last time I checked it in early December.
Interesting @evolved08gsr . Thanks for your constructive feedback, really appreciate it. <3
It looks like @lpfaust posted something above that's a little more detailed. I'm sure there's a decent amount of information on the www about this stuff, with more detailed explanations. I need to do more research myself too on the line-by-line details and requirements.