Will that be a problem though? Don't ghosts tend to spawn near the place of their death, which in this case is about 100 z levels below my fort in the magma sea?
It takes time for them to know that they are dead.
In that time you must have memorialized them on rock slabs. I'm not sure about what type of ghosts though but they do move through walls and are annoying.
Oh you don't HAVE to do it before that time limit. You can do the slab ceremony whenever you want and they'll be appeased. Most ghosts don't have much impact on your fort either.
No, the main reason people hate ghosts is that they are an additional burden on your CPU. Pathfinding without worrying about obstacles? Huge FPS hit. Ok yeah, there are some ghosts that do make their presence felt by the damage they do, but all ghosts can go through walls and floors, eating up FPS.