There's just a slim chance that a king slime spawns instead of a blue slime.
Areas with high slime spawn rates are great for this. Of note are pocket dimensions (if you the dig the whole thing out and replace FoR with something that doesn't give off light) and for those that don't have dimensional doors, mushroom islands. Or any other biome which doesn't have hostile spawns. Blue slimes have a tendency to spawn EVERYWHERE. Heaven(promised land) or hell, pocket dimension or outer space, THEY ... ARE... EVERYWHERE.
They do, don't they? I built a small temporary base in a mystcraft age to use for thaumcraft (so I can go crazy without having to worry about flux) and they are spawning inside the building. So annoying. I guess they are not affected by light levels, just like regular slimes, which means I'll have to use slabs. They don't even drop anything good - gelatinous slime balls are just an inferior version of regular slime balls. But if I can get some goodies off of King Slime, then it's worth it. I'm now considering making a mystcraft age designed to be ideal for slime spawning. Hmm...