Shadowfiend / Mindbender's stats update dynamically since 5.0
The only exception is that casting Shadowfiend / Mindbender before Hero / BL will cause it's swing timer to revert to base. This is surely a mechanism put in place to prevent double dipping but is the only *real* thing to take note of when casting our little face sucker.
Because the duration of Jade Serpent pots are 25 seconds I will reserve using Shadowfiend / Mindbender up to 10 seconds into the pull depending on procs. Breath of the Hydra is HUGE so I generally try to line up ~80%+ uptime on: Jade Serpent, Lightweave, Jade Spirit, Breath of the Hydra and Light of the Cosmos.