Karthus going top in dominion is shit funny. Stay back by capping the closest point, then rush straight on to the point, as the fighting should still be going on there, and just activate your e. They're either forced to back off and give your team the cap, or they have to kill you and thus take focus off your team, plus let you still fuck shit up on the point for the next 7 seconds. It's an almost guaranteed cap for your team if they take advantage of it.
Also, I have yet to lose a game of dominion as karthus. Global ult + amazing early turret offense and great late turret defense leaves me wondering why more people don't use him.