I use cacao nibs in one of my imperial stouts. For 5.5 gallons, I dry-nib with 4oz. I've never boiled it. It's a nice note of chocolate. I roast it in the oven @ 225 for 10 minutes, stirring half way through. I've followed that up with soaking them in enough bourbon to cover and throwing all of it in, although I don't think the bourbon is necessary. I've never boiled nibs.
I tried cocoa powder once in the boil and found the experience messy and the beer to be chalky, but that's just my experience. If you really want to do something in the boil, I've had good success with taking 90% cocoa dark chocolate, something without preservatives, and shredding it on a microplane (to dissolve easier). I throw that in the boil in the last 5 minutes, giving it a good stir so it doesn't scorch on the bottom. I do around 3.5oz. Shredding it on the microplane can be messy, though.
Keep in mind, I'm also relying on some of the malt to provide some chocolate characteristics.