You'll see what I mean by beyond the edges when I get some more of the granite lands tileset finished. I've got a few tiles that put a mountain outside the area edge. I've always hated that graying at the edge, so I'm designing what I call "surround" tiles. Instead of having a single type of edge tile per combination at the edge, you place a terrain type which dictates what is seen beyond the edge of the area. I have to craft my areas (with fog) in a way that keeps the view of the player within a playable space, but yet allows more visual variety.
With the image I posted above, what you'll get is a set of one or two tiles (tile group) which has a left or right spanning region that far escapes the tile 1k boundaries. It will let you place that tile wherever you need it (where it fits) but you can use the escaped area as an edge tile, or in the middle of a long narrow map. It doesn't really matter, just as long as you don't double back on that tile, unless you specifically double back over the escaped area at a higher level, so it gets burried under the floor of that tile.
For the underdark sets, I have a LOT of that planned out, as I intend to have very high and very low places in the caverns, which you can view, but cannot reach. Taking ceiling-ed tiles to a new level '>
Here's another inspiration image to show what I mean. This is from DDO and shows how expansive their underdark region is. In NWN you would have to choose to walk above or below the region, or like they do in TL2, simply show the lower region, but never reach it as you progress to the next area.
And here's another image showing just how good you can make a simple area in your dungeon/underdark appear by just making off-tile unreachable areas. In the background you can see so much more cave, but they never intend for you to leave the 2D area they have planned out. It brings a lot to the game.