I'm only up to mid-Kara, so I'm not as experienced as many other posters here, but I'll throw in my thoughts.
I tend to not use either anti-resist curse for just one person, unless it's a shadow priest. They already have all their +shadow damage, but then another 13%, and a good chance for +20% more? They love me.
And when there's two of them in the healer group, the whole raid loves me.
A couple fire mages gets CoE; an extra lock, a shadow priest, or some arcane mages gets CoS. If it's just me, I finally get to use amplify curse. That ability is so underused, outside of fights like Curator, where I can do 25k damage if I time Doom right.
Just have to agree with Loki. If you're fighting a caster that can have CoT on it, that's either a straight out 60% damage reduction, or a +60% chance that the the spell will be interrupted completely. We made it through an entire Spell Shade pull with 0 damage because of my CoT on all of them.
Edited, Nov 25th 2007 12:41pm by Ehcks