It depends on your job as others have said. As WHM I typically will pop Regen on the tank at the start of a pull, especially on a speed run and then flip to CS, and depending on the mob size, do a presence of mind to speed things up then spam Holy. Love seeing those mobs blow down because of me. If it's a large pull, or I see the tank taking lots of damage to the point I need to hit a Cure/Cure II before I can even enter CS, then I sometimes will do a Medica II to add a secondary Regen, and then same thing spam Holy. If it's a really large pull or tank taking big damage, I'll use Divine Seal first to boost that potency, then pop Regen and flip into CS. For bosses I frequently will hit CS to keep my DoTs up like Aero III, Aero II. As others have mentioned, the 6 second cooldown isn't really an issue since you can get out of it fairly quickly. Worst case there's Benediction which restores 100% of HP instantly even if you have CS on. As you mentioned though, low level dungeons are easy to heal so it's not a big issue. You'll notice higher up though the difference, as every so often you'll think why is this guy so hard to heal, only to realize you forgot CS on.