thats a really generic answer, some cars are not unibody and are welded together, and having just a ground spot in the back will cause horrible voltage drops, it all depends on the car, most of the time it is fine, if you are doing a huge build with lots and lots of power then its worth it to tun that negative wire back there, you will not only get steady voltages, but will be better for your amps as they dont have to deal with a shitty ass ground, also you have to have the same size ground wire as the positive wire, so if you use a distro block with lets say 3 8gauge wires coming out, then you need to make sure that you positive going into the amps are 8 gauge, and if that power wire is to small then you need to upgrade both neg and postive to bigger wire, it has to complete a curcuit.
Yea it was "generic" but to the point i doubt hes running crazy amounts of power so i dont think it would matter in his situation although it might i was just letting him know that he didnt have to do the negative that way he could just ground it in the back and as long as he had 0/1 awg wire goin to the distro and no more than a couple 4awg or 3 8awg it should be fine imho