You don't need an index to select rows based on the contents of a json document in 9.3. You may want or need them depending on your queries. (I use them sparingly, the most common pattern for me is that an indexed non-json column filters a query down to a smaller set of rows, and then a further index on the attributes referenced in the query isn't necessary.)

And if you do want to create an index now on a JSON attribute in 9.3, a function isn't necessary.