My experience is that, outside of near death situation, IB should only be used :
- when you're down enough so that 2 heals won't top you (basically when you're below 50%)
- And you either have Infuriate or Convalescence (maybe featherfoot) available

Healers really feel when we don't have the heal buff (more than our IB...) so we really have to keep it up as much as possible. If IB is used when over 50% of HP, there's high chance that we overheal by a lot.