ARSMAINT has two main modes -d (data expiry) and -c (cache expiry).
The -d does document EXPIRY (careful if you never ran this) as it will purge production documents forever !!
You have also mentioned "All of our archives reside in cache." Not sure if you mean using Cache as a primary storage;
in which case the cache maintenance "-c" will not do anything
or
each AG is also cached while using other primary storage e.g. TSM or a Cloud Service. In this case the -c "The cache maintenance" will purge cache only while keeping the document(s) in the main storage area, and it is safe to use.
Otherwise, as Justin mentioned run it first on a small AG first, ideal in a test env and review system log afterwards, also it is good take df -m /my_cache_dir before and after so you could see how many MB was released.