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
what if im running two different brands of wire to the back, would it make any difference? they are both 1/0 gauge and ofc but the - is streetwires ultra flow and the + is jl audio metawire