It also depends on the country. In Slovenia, cryptocurencies are not money and are not stock. So you don't pay anything if you buy and sell for more. It is "valued" as a moving object, the same as vehicles.

But things get complicated a bit, if you mine them, than you didn't buy and sell for more, but you got paid in them for "labor". So you are taxed the same as any income. And i assume that it might be the same with steemit. But as far as i know, they track you when you make a bank account deposit, but I've read some forum posts that mining should be actually reported every month. Problem is that as far as i know, we don't have a mechanism for reporting that other than opening an business.