Hi Jonny,
Since you are probably going pwm for a controller, the wiring that matters is from the controller to the battery bank, unless there is a "battery voltage sensor wire" for the controller. (Tristar does have battery sense wires--and so does Rogue)
Just as for an inverter, the controller, in an ideal world, will be as close to the battery bank as is humanly possible without it being in the same compartment. And again, just like an inverter, the size of the wire from the controller to the battery bank is better if it is oversize.
To put it another way, for pwm, voltage drop from the panels is mostly a non issue, but voltage drop between the controller to the battery bank is mission critical.
How many watts are you planning on?