Digging a straight up/down stairway is dangerous, because, if any dwarf happens to fall, they'll fall all the ways down, ending in a bunch of red ² at the bottom. And everybody know that crushed dwarf is hard to clean.
I would rather make your pit with an alternated staircase on the side, like this:
HHH level Z=0
HHH level Z=-1
Like this, if a miner happen to fall, he will just for for one stair, much less dangerous.
Also, you can dig your staircase first, it won't matter, but make sure to pit the pit from top to bottom, a floor at a time. Since channel automatically create a ramp on the floor below, your dwarves won't get stuck if they happens to fall while digging, if the floor below have not been dug at all.