Ok, I main Karthus a lot because of his general dependability in a team fight, and the fact that he can do more team damage than almost any other champion when used correctly. People who complain about other champions being gear dependant don't have much experience with Karthus.
Unless a kill was 100% guaranteed (when is this truly the case?), Ult away as Karthus. Do what you think is best in the situation, if you won't need it, use it. If you want to save it, then save it.
Karthus takes a LOT of gear (thus a lot of gold) to be good, or even decent. He is as vital if not more so than the ashe, or trist carry. His ult can do 1000dmg, which is amazing, but his true value in a team fight is defile. I can get it to tick for 150-200dmg a second, and using wall to keep them slowed (and rylais), can usually shred a tankish team for my teammates to finish.
I almost always die in a team fight because even with RoA, and Rylais, Karthus is squishy, but his passive + defile + Ult mean that he still has 8 seconds to destroy the enemy.
In this manner I play a sacrifice Karthus, and it often leads to an "ace" with 4 of my teammates up, and me 2-3 kills richer.