When drywalling a room, should I do the ceiling before the walls?
If you do the walls first, you can end up with drywall that is unsupported along an entire edge.
Taking these walls for example.

Walls First
If you add drywall to the walls.

Then add drywall to the ceiling.

The ceiling drywall will be unsupported along the entire length of the one wall.

Ceiling First
Given the same walls. If you add drywall to the ceiling.

Then add drywall to the walls

The ceiling drywall will be supported by the wall drywall.

Blocking, or Extra Framing
If you add blocking; or some other framing, it shouldn't matter which you drywall first.