The tax situation could be a nightmare especially if you have multiple altcoins and frequently buy and sell. My understanding is also that cryptocurrency is currently considered to be like a stock, so you would need to pay capital gains tax if you sell for a profit. Of course, you might not even *have* records of all your past trades - depending on the exchange it may not even keep the history, and almost certainly doesn't provide you with tax documents, or the exchange may not even exist anymore at all. With cryptocurrency you can also incur permanent loss or stolen coins and have no way to prove to the government that you actually lost the coins. Also if you buy any good or service using crypto, technically that counts as a sell, so you would need to calculate capital gain/loss for everything you ever buy with a cryptocoin. So yeah, it can be messy. In the meantime I am kind of not selling any coins for fiat in the hopes that it will become clearer in the future how to handle the taxes, or that the government will start actually treating it as a currency for tax purposes.
The best I could hope to do at the moment is to obtain the history of all the bitcoins I have purchased, and then if I want to sell any crypto, I would convert it back to BTC first and then sell and then match them to a previous buy to get a cost basis.
If you're a small fish and you make an effort to not evade the tax, the IRS probably won't really have a reason to make an example out of you. But if you're a big fish and you anonymize your coins and don't report large gains, you might get away with it but you're risking probably years in prison...