You don't need to purge the carboy with argon or any other inert gas. Sanitizing the carboy will be sufficient. Purging the carboy is done more to avoid oxidizing the wine (or cider) as opposed to avoiding infection by spoilage organisms. Provided you've been using sulfite as required and have been sanitizing your equipment well, you'll be fine. On the other hand, if you have concerns with the amount of oxygen exposure your cider has seen or you smell signs of oxidation, or if you just want to be extremely protective against oxidation, you could purge it to avoid further air contact.