On multi engine planes, the biggest factor about the location of the engines is what happens when one quits. On under wing jets like the B737, the airplane wants to turn towards the side of the failed engine and the pilot must counter with opposite rudder. The farther away the engine is from the center line of the plane, the greater the turning force when one fails and the opposite side is still running. This is most critical if an engine quits right as the plane is lifting off the ground. On planes like the B727 and DC9, which have the engines on the side on the tail, the turning force is much less when an engine quits.