If you use a relay with the ignition, you can ensure that you never forget to turn the switch off and come back to a flat battery.
I've got a relay that controls a fuse box in the trunk wired directly to the battery (with a fuse as close to the battery as possible). From the fuse box, I power a Kensington 120W laptop adapter (which was spliced into to get a 5V line for my external soundcard), another 5V power adapter for my USB hub (from a 2W USB cigarette lighter power source), my amp for the front and rear speakers, and my subwoofer.
Otherwise, it looks good
Cheers, Marc