I wouldn't use the 12V socket as this is fused and if you blow the fuse on this, then you won't be charging the battery. .
That's how my GS is currently set up, but I need the charger to attach to it..!!
If your connecting directly to the battery and the connection is more or less permanent, you don't need to switch the power to off, just don't connect a live feed, connect first then switch the charger on. Or you can switch the power off if you want..??
Don't really know of any other car that comes with a battery isolator as standard fitment.
Definitely handy though.
For those system resets!!!
For those wanting pretty lights, install the LED Fuses, when they blow they light up, so when the car stops in the middle of nowhere in the dark, you will have pretty lights on to cheer you up..!!