Choosing decorations has come up before. Material choices are probably something the player can pick. This is for more than that- about what dwarves engrave, decorate, etc. -you probably don't want your gold statue studded with brass.
Why they shouldn't be wholly uncontrolled:For Urist McLegendary's tomb, you don't want your cheese-loving engraver to coat it in images of cheese and the election of Kivish McMayor. You want images of Urist McLegendary and events from his/her life, and things of their preference engraved.
For decorations, you don't want images of axes on that bag of seeds you're trading to the elves (Well, probably you do, given the dwarves around here, but this should be something that is up to the player. Random "screw you" just because you failed to thoroughly inspect your decorations on containers shouldn't happen.)
That said, while I think you should be able to "stud with silver" and order engravings possibly in a theme (of Urist McLegendary, or food or just high-profile events for the dining hall) you probably should allow them to pickwhat they want- what was significant to them about Urist, or their favorite food, and so on.
edit: Bother, I just rehashed
this in some ways. Linking as rele'ant, adding more.
On the other hand, if the engraver doesn't like the person they're carving a tomb for, they might think "Oh, Urist hates fire snakes." and, how I think it currently appears:
This is a masterfully-designed engraving of Urist McDead and fire snakes. The fire snakes are striking a threatening pose. Urist McDead is in the foetal position.Also, as some people like to carve halls of history (or try, I don't know how successful this is) perhaps order carvings/decorations/images of season/year, or "major historical events" (see dining hall)
More ideas: Some engravers might want to (or might be ordered to, by power-mad warlords- not necessarily in your forts, but possibly) be ordered to produce engravings of events that didn't happen. Embellish a legend, remake it to suit the prejudices of today, make Human McWarlord seem the victor of a thousand battles with nary a speck of blood on his surcoat...or, snazz it up, as dragons fly low over the grime-encrusted battlefield, he bats an arrow from an offending archer with his shield into one of the dragon's throats. Sort of inline with
war stories and other knowledge suggestions.