One of the ways to look at it is with Mistlands released you can fully explore a world seed without fear, staying away from the far south and north and that seed will be good for when those biomes are released.

The only thing I worry about is when the Ocean is finally updated (my favorite and most eagerly awaited roadmap release) as I'm not sure if it would work the same with 'undiscovered' areas updating with successive content or would Oceans require an entire new seed.