I have been messing around with cacao nibs a lot in the last 6 months. I have always put them in secondary (usually tied to a keg dip tube in a vodka soaked disposable hop bag). Yes, gently crush the nibs before placing in the bag.
I find that the crush + Vodka bag help to pull out more flavor of the chocolate into the beer. A min of 4 weeks is needed. Warm or cold didn't make too much difference as far as secondary temp controlled.
The last thing is that the chocolate flavors really opened up more in a darker and higher ABV beer than lighter ABV and color beers.
I have worked on blond ales, american ryes, and Baltic porters with nibs. Makes sense to me, but the Baltic Porter was the best!
Remember, Cacao nibs are not chocolate. So plan the beer to have a nice residual gravity or a big mouthfeel (rye really helps here) to help give the "chocolate flavor" more push in your finished beer. Early tastes of the nibs will give you an earthly aroma and flavor, but tends to die out over a month.