Oh yes, forgot to mention -- dwarves with material preferences only demand that particular material if you've smelted it yourself.
Seconded and expanded on: Always check your dwarves' preferences before assigning them to metalworking professions. If you're going to be making steel (which is pretty much a given), if you have a smith who likes pig iron, you
will be getting a pig iron somethingorother, no doubt about it*. If the prospective smith likes something you'll never have a reason to smelt on-site, like zinc or pewter, you can use forbidding to force the dwarf to use whatever metal you want. If the dwarf already likes steel or candy, make damn sure he's an Armorer, as (IIRC), dwarves working with their chosen materials get higher % chances of making masterworks, and you're going to need a lot more armor than you will weapons.
* Dwarves
can sometimes be fooled into using a different material,
if their artifact requires a second item of the same type as the base material. If a dwarf requires "bars, shining . . . tree, life . . . bars, shining . . . bones, yes . . . rough, color," then you can let them take their chosen metal (nickel silver) to the forge, followed by some wood, by which point you will have {forbidden} all of your metal bars except steel. He brings the steel to the forge, at which point you forbid the nickel silver and the log. This puts the steel bars in the #1 spot for the artifact's construction, and as it's valid for the forge reaction, the artifact's construction will continue--once the dwarf grabs another metal bar & wooden log, to replace those that inexplicably vanished.