It's no different than pistol rounds, you can either crimp in one step using the seating die or in a seperate stage. I usually use a Lee Factory crimp die (so much controversy surrounds this die!) to put a very light crimp on mine. Just like with pistol ammo, you'll find people who don't do it at all.
I find that I have to do it on my semi-auto .308, otherwise, I have setback issues when the round contacts the feedramp.
It's just another reloading variable that needs experimentation.