It also fits in with the idea of the hidden bastions of the dwarves-which is pretty common in fantasy. One does not simply waltz into the mountainhome-one has to figure out where the place IS in the first place! It should, however, take a very long time for even a legendary dwarf to do, and should be part of either masonry or engraving-or require both. Something like that is a work of legend, to say the least.
I would like to be able to fill in areas already dug/channeled out. Even though constructed walls would be made from bricks and blocks of stone, the facing sides of these stones would be extremely smooth.
I just went to the park today, and the stone grilling area was constructed from large granite blocks with concrete or something similar used for mortar. The surface of the actual bricks was nearly mirror smooth, and the only rough areas were the mortar cracks and a very small number of rough granite blocks. This was built by human beings, but I have no doubt whatsoever in my head that dwarves have perfected a mechanism to have concrete or mortar for their one walls, and have it applied in such a natural manner that even their mortar cracks are smooth!
My point with the long-winded previous paragraph is that constructed walls, stairs, doors, etc. should easily be smooth enough to engrave upon. However, a continuous corridor of naturally smooth stone is an impressive sight to see engraved. A natural cavern or hall, hewn smooth by the skilled hands of the dwarves, should definitely be extremely higher quality than engraved constructed walls.
Also, filling in with soil and clay and loam would be very neat. There's another thread that has been floating around about pottery, and seeing clay and soil collected in bags like sand is epic win. What you do with those materials: gardening, crafts, etc. is just added bonus for them.