If I understand your question correctly, you are asking if it is necessary to run another ground cable from your relocated battery in the back all the way up to the engine compartment? If that is the question, the answer is NO. BMW, Mercedes Benz, Lincoln, Maserati, etc don't do it so neither should you. Your car's body is one big metal ground (wire).

You do however have to run a ground strap from the body to the engine block as your starter, alternator, and many engine sensors need this ground connection to complete their current flow cycle back to the battery.

Make sure also that your ground from the battery to the body in the rear is REALLY good. No paint whatsoever between the ground and the terminal and the terminal must be tight. In race cars and 4X4's, I have seen people go as far as tacking the ground terminal to the frame/body with a welder just to ensure that it remains tight even through bumps and time.