While not a rule, some people prefer the cannelure if they crimp. Some people claim its gives greater neck tension so the bullet doesn't get set back when shooting out of a AR platform. I have never had a problem with it, to be honest...
Some people claim a crimped bullet is slightly less accurate than a non-crimped. I have never tested this in my load development...
Bullets for ARs get a medium crimp, bullets for non-ARs don't get crimped.
