Do Minecraft monsters spawn on stairs?

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: