I think I would only bring earthworms if I wanted to completely nuke the native ecosystem. Ecch. I'm going to end up with invasives one way or another, yeah, but unless I'm missing something, if the planet has a earthworm-equivalent already we won't need to bring one, since most of what they do is soil aeration and simple decomposition, and if it doesn't have an equivalent, then they can really mess up the existing ecosystem, quick. (Oh, my lovely North American forests, and your sad shadow of what you once were.)