I add it to head.php… I understand enqueuing scripts via Wordpress, but that is really so that Wordpress is aware of scripts and they are not loaded twice. I don’t see Google fonts as a Javascript or Jquery plugin, so I just paste in the code in the head.
So basically, you would probably continue to enqueue it in Wordpress if that works for you. But since you are loading it from Google and not your own server, you can’t really minify and concatenate it.