Your #1 job as Karthus is to get off requiem during EVERY teamfight. That means having the 40% max CDR is absolutely amazing as it lowers the cooldown on your ult from 160s => 96s, thats fricking over a minute in cooldown reduction. I can't emphasize enough how essential that 1 min cooldown reduction is to any serious Karthus build: your job as AP carry is doing damage, and that 90% of that means getting off requiem.

With this build, 40% CDR is achieved on the 4th item.
You get 4% from masteries + 1% from runes + 20% from Morell + 15% from Deathfire = 40% CDR

Karthas has 2 zoning abilities: Wall of Pain and Defile. Wall of Pain is great at forcing your the other team to commit to a fight or to disengage. Defile does crazy amounts of damage and punishes enemy melee carries trying to blitz you. Given your passive, it's important that you use defile offensively in team games by staying close to their melee. Especially when you the enemy team is heavy on melee champs, your defile will wreck them.

Your passive gives you an interesting choice for approaching team fights. If you expect to die, it's best that you be one of the first to go. That allows you to get off a well-positioned defile placement for your passive and safely cast your requiem onto all five of the enemy team. If you expect to live through the fight, it's important that you stay back and start channeling requiem the moment the enemy tank commits and then support from the back. The worst thing that can happen is if you start channeling requiem in the middle of the fight and then get silenced/killed, which cancels your requiem and puts it on cooldown. It's also bad if you cast requiem after 2-4 enemy champs are already killed as that makes your requiem 40-80% less effective.

In teamfights, your requiem should not be getting last hits (as much as we all love to kill steal). It's much better for you to use your requiem to soften all five of the enemy team up for your bruisers since requiem is guaranteed to hit all 5 when you cast it at the beginning of the fight, but it certainly won't hit all 5 if you cast it at the end of the fight.