A basic rule, which every player should know, is that preventing damage is almost always better then healing. Every time a player avoid's Rotface's Slime Spray they have avoided 5K of damage which otherwise would need to be healed. Healers (and Raid Leaders) should look at the damage taken meter and have a private chat with any DPS or Healer that seems to be consistently near the top of that meter.
I say almost always better, because you can't rule out situations when players do too much to avoid damage and end up not doing their primary jobs. In most encounters damage must be taken for the raid to be successful otherwise Big Heals would be a very boring job.
Improved Righteous Fury reduces all damage by 6%... I have RF up whenever I can get away with it. It's good not to have RF up on fights where you might pull aggro from adds. For example, the Big Ooze can be a problem during the Rotface fight. You don't want the Ooze tank to fight you for aggro or even worse pulling aggro and having the Ooze running into the center and decimating the raid. But this fight does not have continuous damage like his brother Festergut. During the Festergut fight, keeping RF up will mean you will personally need 6% less healing and that is a good thing.