Given @suheb's recent updates to the cltk_frontend repo and how we've already entered into pretty complex contracts between the API and frontend app, I think unless anyone's against it we should start versioning our API in the cltk_api.
I think the small amount of additional complexity here would outweigh the longer term benefits.
Would setting Accept headers be an alright solution for this? Can create an issue on the cltk_api repo. Looks like this is pretty easy in Flask: http://flask.pocoo.org/snippets/45/