If you rev your motor past 9k I highly recommend one. I have had two instances where my crank sprocket bolt came loose just enough causing my trigger plate to wobble and hit the crank sensor.
These instances both happened after a few thousand miles of street/highway abuse and high rpm shifting.

FWIW I have a fully balanced and blue-printed rotating assembly and bolt was torqued to spec.

 

I can vouch. I was there when it happened.