At first I used 3x3 up-down staircases. Then I got the idea (in 40d) that people were falling down them because I found dead bodies at the bottom. Whether they were actually being killed by the fall or being killed by something else and then falling, I never did find out. I suspect the latter.
So I began making spiral staircases, with up and down staircases in pairs alternating around a conceptual middle point. This was a pain to designate.
Then I got the ramp bug and started building spiral rampways, 3-wide. Some had solid cores and some had open wells in the center.
Enter 31.x, and then 34.x, and I am back to central up-down staircases. I use a 5x5 to bore straight down to the magma layer, or at least as straight as I can. If I can go perfectly straight with only a little patching of the walls, I do that. Otherwise there may be a horizontal jog in the path to the forges. I find this preferable to pumping magma up 50 or more levels. My fortresses now are much more vertical, with only a horizontal spread towards running water, soil, and the depot. Water gets piped in. Soil, well soil is where it is, for farming. The depot is in a trapped squiggle of corridors with drawbridges to isolate it from the outside and from the fortress, in whatever combination may be necessary. Sadly, I have not yet mastered making murder holes. The last time I tried to retrofit a drowning capability onto my depot area, the dragon trapped within ran RIGHT UP the ramp that appeared when I "channeled". *sigh*