Although you can design your fortress to dump refuse into the magma sea, it's probably less trouble to instead build a Dwarven Atom Smasher. Atom Smashers have the advantage of not requiring a path to the magma sea; they can be built anywhere. To build one requires only a lever attached to a drawbridge (two mechanisms are also required), where the open drawbridge lays flat on one or more tiles where you designate a dump area. When the drawbridge is opened, it smashes all items under it into oblivion.
Advantages of using an Atom Smasher (rather than the magma sea) to get rid of trash:
1) Avoid the 'fun' of creatures that live in the magma sea entering your fortress.
2) No need to create a chute or other way to get refuse down multiple Z levels to the magma sea.
3) Artifact items may not be destroyed in the magma sea; these undestroyed items will lower game FPS since the game still tracks them.
Disadvantage of using an atom smasher:
1) One of your dwarves needs to pull the drawbridge lever now and then to destroy garbage.
2) Dwarves standing too close to the dump point may be obliterated when the drawbridge is dropped.