You can.. or anywhere the coil will not get B+ V....
If the reluctor on the Pert happens to be stationary in the trigger position by chance with B+ feed , it is energizing the coil and will burn out. But, if coil is out of the circuit, there is no high Amp draw on the Pert Module.
Very common on the Pert-1 , the Pert 2 has a breaker circuit so as to allieviate that condition, but guys like the performance and simplicity of the Pert-1 better., so a kill sw is the answer. It also comes in handy if you are working on the car and need the key On for testing or adjusting valves, etc without having the key power the Ignition.

And if you hide that sw under the dash, [ or anywhere] you automatically have a theft protection Kill, although that is not the intended reasoning behind the switch..it is just an added benefit.
Some guys also go a step further and use a DPDT sw so they can also add a Blinking Red LED to the switch ....they mount the LED in an Obvious spot in the Dash so any passer-by crooks see you have an ARMED car.
No big deal, but better than nothing...The Led also reminds you when you go to start the car that you have the Kill ON.
Radio Shack has a cheap 12V Blinking Led that has the blink circuit and dropping resistor circuit all self contained, so you just have the 12V +/- to hook up to have LED Warning. Real simple.
Some guys just do the LED ..it does nothing but look like an Alarm..I put one on my Golf Cart....

Don't put the kill before ballast b/c you will still have ign with crank if the car has Ballast By-pass for Start/Crank...........................

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A Dalton