In my builds, especially the bigger ones, one of the most difficult things is to get everything symmetrical (that needs to be). A while ago I made a Bukkit plugin for personal use (on a private server) that would create blocks around specified axis in-game, and I think there should be a mod that does that (I have way too much other stuff to try and do it myself). I don't think it would be too hard, it wasnt very complicated when I did it. Of course, it would probably be used for creative mode since it makes blocks out of nowhere, but I think it would really help the more complicated decoration on bigger builds. I'd be happy to provide the plugin code I made if needed as well.
How I had it is, you use a command to set the center block and add options telling it if you wanted it to mirror on the x, y, or z (or a combo) axis. Then youd place a block and it would count how many from the center block it was in one direction, and then add another one that far from the other side of the block. You could make really easy spheres, domes/circles and all that, and tweak it until it looked right without tearing it all down to see how another way would look.