Are you having problems with something? You're going to see an improvement or power gain just from replacing ground wires. If you're having a miss or odd engine problem, sometimes adding a ground can help, particularly if your old ones are missing or corroded. It is true resistance can increase in a wire with heat time and corrosion, but replacing battery wires isnt exactly routine maintance. Check the ends for strain breaks and clean corrosion and you'll be fine.
Replacing the wires isnt going to do much more than just cleaning the connections would. The problems usually occur where the connections are, moisture gets in there and corrodes the metals. If you're really desprate, just add a ground from the battery to the strut tower.
The main wires you need to be concerned with are the following
Ground from battery to engine. This is on the front head on the V6 ?4cyl
Main power from engine to starter
Power from battery to distribution block on the right rear fenderwell (just below C500)
Power connection on the alternator. There is also a soildered splice about 8" up from the alternator connection, Ive seen this corrode out.
I think there is another ground off the battery too.
If you really need to, there is also a main cocpit ground under the center console, on the passenger side a little up from the rear firewall.
[This message has been edited by 86GT3.4DOHC (edited 01-23-2008).]