Hi,
Active/Active just means both servers are up and capable of taking calls. Best practices for UC follow the same as CUCM, where the subscriber does the majority of the call processing and the publisher handles all of the client access stuff (PCA, IMAP, etc). Nothing is stopping you from sending calls from both subscriber and publisher, however the documentation says to send all calls to the subscriber. If that ever becomes saturated, it can overflow to the publisher without any end user impact.
Hope that helps,
Brad