I guess it all depends on how productive your mining locations are. If you're finding lots of iron, go ahead and make picks. My first one was pretty sparce, so even with an iron pick I couldn't replace the iron fast enough to justify it. And I needed the material for armor. Easier to just create an armload of cheap stone picks as needed. Doing a lot better on my second shaft. Rail lines nearly done, and I've got about 30 pieces of iron lying around.
"The way I see it, every life is a pile of good things and bad things. The good things don't always soften the bad things. But vice versa, the bad things don't necessarily spoil the good things or make them unimportant."