I think there's a better way; keep the concept of plane vague, but detail the stuff you're really interested in. In my campaign, when mortals die on Staunwark Island, they either get carried off to the Rest Eternal (heaven) or get caught and dragged down to Beyogo's Indenture (hell). The point is, because I kept it local, I can just keep mapping and developing those realms, and be confident that they'll fit into almost any planar system whatsoever. I think for Planescape we're looking at Bytopia/Carceri, which lines up nicely, but that's not set in stone. To this I've added Shalanam (a sort of mountainous heaven for Arcana Unearthed giants) and The Tomb Beyond All Reach (where the dead gods lie...). Thus, there's somewhere important for plane-travelling heroes to go, but I don't commit to any particular cosmology. While I know that 'cosmology' is supposed to be something fully defined for a campaign (according to Ms. Manners' guide to setting design), focusing on what the PCs can experience has helped me make my campaign a lot richer.