My campaign world is enormous- it takes place on the inside surface of a bubble of air in an infinite ocean; the bubble is about 780,000 miles in diameter. So when I started it up, I decided that I'd limit teleport to 100 miles/level and teleport w/o error to 500 miles/level, and succor (nowadays it's called refuge I think) to 1000 miles/level.
It's a long enough distance that the teleport spells still let the pcs go just about anywhere they've been before, but not anywhere in the world, which allows me to keep lots of unknown things out there.