Depends, i often go for between the engines on a wing, that takes them down often but it takes time, no instant crash so i often only get assist points, or i aim for the elevator, once i see enough pieces break off there is little hope to land it safely. No point littering the hull with bullets from behind, it will get you killed in no time and has little effect. Also when you attack it is best to have at least 4 x 20mm.
On a rare occasion i spray the hull from the side, front to back, often you hit something important then.