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.