Hm.. actually are they incorporeal AND massless, or just incorporeal?

If just incorporeal, you're right of course that initially after death they'd be falling back and forth, but over time through uneven gravity/curvature and through heat loss (stretching of unequal acceleration applied across the ghost essence, potential energy conversion, yadda), they should generally settle to the center after some time, unless there's a maximum natural pressure of ghosts at whatever temperatures they have, so they may spread out somewhere within the crust if there are enough of them.

If they're incorporeal AND massless, then I totally F'd up and they'd fire off at light speed as soon as they shed their mortal coil.