When do you use renew? ALWAYS! lol
Renew is usually the first heal I cast on someone, especially the tank in an instance. As it is a "heal over time" it does not cause an instant amount of threat, but rather builds my threat gradually, allowing the tank to increase HIS threat on the target. I try to always have renew ticking on the tank in an instance.
In a regular group situation, I'll cast renew on someone when they hit 85% or so if the fight is still going on. Then I either wand till the mob dies, or heal more if I need to. I look at renew as a "stablizing heal" it won't mitigate all the damage someone is taking, but it will usually keep their health from dropping like a rock, and it will give you time to cast greater heal when you need to.
It's also great to cast on yourself while fighting a mob rather than using your Power Word Shield. It costs less mana and will generally keep you alive through a fight.
Hope this helps!