If you connect to battery it will always remain on. If you want the outlet to turn off when bike is off, a relay would be a better option.
Personally I like the gps to remain on so it doesn't cycle every time you turn bike on and off. Also make sure your usb has enough amps to run a tablet, I was unaware there was a difference but noticed some of my droids wouldn't charge for beans with a random plug. Also having a live outlet allows you to quickly attach a battery tender through the 12v plug
But you need to remember it's always on or you can have a dead battery especially with something that draws a lot like a tablet.
We're not out here to rough it. We're here to smooth it . Things are rough enough in town.