I find Inner Beast is best used as a counter to a heavy damage spike. For example, after Titan lands for Geocrush. You can Inner Beast and heal yourself for most of the damage, leaving the healer more time to top off other people.

Ideally, you would want to "prep" your skills in such a way that you can gain back your Wrath stacks asap. Assuming you are able to prepare of course (not always possible) you would make sure SS is primed and ready to go by using Heavy Swing first, then blow IB when you need to heal the damage followed immediately by an SS/BB combo for two stacks of Wrath.