There is no safety reason to install a shut off switch unless you are racing. If you drag race your car track safety rules require a battery shut off switch located outside the car. This is so if you crash, track officials can shut off power to the electric fuel pump and mitigate the possibility of fire.
A battery shut off switch is a useful theft detterant item. That is why I installed it. With the battery in the trunk, you are wiring a long stretch of cable, which makes it convenient to locate a switch, while you are doing that. I have quick release terminal ends on the battery cables at the battery, the shut off switch, an ignition disable switch and an adjustable rev limiter which I can set down to 1000 rpm when I turn off the car. These, I hope, will keep the car in my possession.
It's your choice whether to install a switch. For safety purposes, the money would be better spent on upgrading to a gel-style, sealed battery, like an optima, which would avoid spills and vapors by your gas tank.
good luck.
sure it's fun, but it's only a car.