I'm building a house and using stairs for the look of the roof. Will monsters spawn up there at night (assuming I'm sufficiently far away)? Do I have to put torches up there to prevent that?
You do not need to put torches up there. Passive mobs spawn on grass only (at least in grassy areas...), aggressive mobs spawn on cube-blocks only. No steps, no slabs etc.
Edit. As mentioned in comments, monster can be spawned in upside-down stairs, at least in 1.4.6 version (which is old, though)
Here you may check it for 1.8 version:
As of Windows 10 Edition, today a skeleton spawned on the roof of a librarian's house, which is made of wooden stairs only.