I reckon, imho, that what the dwarves engrave should be dice rolled.
Having the ability to surround your dining hall with a story that plays out, read left from right around the room would be fantastic, but Urist should decide to do that (if his skill is high enough) not you.
However. It's a bit silly that Urist decides to engrave an awsome picture of your Legendary Axedwarf slaying a Dragon... in the pump room.
What I'd like, is occasionally, really really skilled engravers get "inspired" by something and pester you to let them engrave something awesome.
For example, Urist sees a hunting dog rip the head off a gremlin, and he thinks "Yeah, that shit is something special." So he goes and petitions your mayor for somewhere to engrave that scene. And you pick where he puts it.
So Urist has a really good Dwarven Wine biscuit, and wants to tell the world, so you put the engraving of "Urist McEngraver and the Biscuit. The biscuit is being eaten. Urist is giving everyone the 'O' face." in the dining hall... or Urists room.
Also, maybe set an area for your good engravers to cruise around and look at stuff. Like maybe on the outskirts of a battle. Sort of like a photo-journalist... but for dwarves... with stonework. Whatever. You get the idea.
This way, everyone gets what they want.