battery into boot... (no not another How do i do i)

[Stealer Of Souls]

definitely no need for running an earth back to the front of the car, the chassis can carry more current with less resistance than the wire you're gonna use (unless you are stealing it from buried high voltage cables)

IIRC chassis can (but not always) present as much as 1ohm from bay to boot. About the same resistance as a length of 4GA wire of the same length...
Bay to bay earth resistance is in comparison negligible...

[sergei]

I doubt that chassis has that high resistance, it is the connection usually the problem, compare cross sectional area of chassis and your wire...There is no way chassis will have 1 ohm, unless it is made out of composite materials, then we are looking into MOhms.
There is no need to run extra earth cable to battery, if you have a good connection to chassis at both points, there will be no significant current going through that cable due to low ressistance of chassis.
But it is actually good idea to have earth from battery connecting in multiple point on chassis, thus removing the bottle neck of the bolt/terminal/thread small surface area.

There is only one advantage of running earth multicored cable from battery to engine bay - and it is RF, due to nature of RF (radio frequency) it propagates through cable at it's surface, in this case to carry high RF currents you need lots of surface area not crossesctional area, thus the cable (if right) would be superior to chassis for this purpose. The main RF source in the car is ignition system. But good designed ignition (wich are the most of these days) will not produce significant RF any way so the cable would be overkill.