Destru:
IC and Soul Rot are useless, as they don't interact with the spec at all. IC's viability depends on its scaling, but overall it's just a button to press that does not change the way you play this spec.
Tithe seems very interesting as a concept, 5 Shards can be powerful for Destru. Sadly Chaosbolt seems really undertuned right now, so maybe Tithe is more of a AoE choice to buff Rain of Chaos. Tithe also interacts with Havok, so that's something to keep in mind.
Decimating bolt seemed the most reasonable choice until last week as it worked well with the "fire destru"-build. Now, we'll have to wait for final tuning.