No specific actions need to happen prior to the moment the customer takes a system storage flash copy. MIMIX does not need to be stopped, but also MIMIX does not need to be active.
However, MIMIX should be configured to a point where it is ready to be operational if started. The MIMIX configuration should have the correct IP addresses identified and license keys applied. Also, the configuration identifying the objects and data to replicate must be identified in configured selection rules (data group file entries). The configured information should be up to date, or at least not be too far out of date.
It is therefore best if MIMIX is simply normally running and is relatively error free at the moment of the storage flash copy.