Apparently we have a new standard!
Take a look at this post by @dan if you haven't already.
So the way this works under the hood is that the client submitting the comment transaction to the network adds a "tags" field to the JSON metadata which includes an array of all the tags in the post (excluding the default tag which is the category name). So @dan's post was submitted to the steem category so it automatically has the steem tag. But because he also included #tags and #steemit in the body of his post, the client automatically added the following into the JSON metadata:
{
...
"tags": ["tags", "steemit"],
...
}
I think this should be useful information to people like @xeroc who are implementing alternative Steem clients.