I may be the last to have figured this out, but in case I'm not: When you're trying to make a large world easily, it's no longer necessary to zero-edge your terrains in the terrain editor, because of one little checkbox in the replicator. Start by creating any terrain; you can "rescale terrain to fit working box" or not. Return to the assembly room. Insert a replicator object by choosing the second icon in the replicator menu and clicking in the window. Double-click the replicator to enter the editor. Choose auto-grid. Under replicated objects, click add and choose your terrain object. (You can also do this in the assembly room, in the "Instances" menu, by dragging the "terrain" instance onto "replicator", whereupon it will place itself under the replicator instance. Look at "assembly".)