RE: U.S. Taxes- you might face big penalties if you don't file FBAR reports
Yes, you would have to report it, as it is the highest balance that counts. On the plus side though there would be no tax due in that case, just reporting requirement.
The only exception I would think is if you only had accounts at the US brokerages, like Coinbase and GDAX. I think an argument could be made that this is US source accounts, especially as they have to report it to the IRS now per the subpoena. In the lawsuit they were only asking for info on people with over $20k in transfers or trades though, so that is who will likely get audited in the next year or so. Still you pretty much have to trade on foreign exchanges if you want to play with anything other than BTC, ETH and LTC... Still though it is safer to report even this account as there is no tax liability, just reporting requirement, and I prefer to overreport on FBAR just to safeguard against those outrageous $10k penalties!
I would argue that you're not actually holding anything in an exchange. It's just a number they keep track of for eventual withdrawal. It only becomes a value on withdrawal. That's how you can make quick trades, and avoid larger fees.
What they actually see it as though is another matter.
Bittrex is based in the US, and lets you trade in many different cryptos, including Steem, which is why I use it. It's a little slow at times though, since it's browser-based.
I haven't used Bittrex because I am trying to only trade mobile as I spend my whole day staring at a computer screen already... My friend showed me an app yesterday that might fix that though, so might be in the future.
Also, it could be argued by the IRS that any crypto currency is essentially a foreign account and thus should be reported: https://www.goldinglawyers.com/fbar-bitcoin-2017-2018-important-cryptocurrency-reporting-tips/
If they try that, it will likely end up in at least one huge lawsuit.