Cold steeping, when to add

I've read through the endless arguments about the various pros and cons of cold steeping dark grains. What I couldn't find was a good consensus on the appropriate time to add the liquid and why. I would prefer to add preboil to make my hop utilization calculations easier. I've read that some people add the liquid at flameout but didn't really explain why. Without the actual grains in the hot water, it seems it would make no difference. Does anyone have any experience/evidence anecdotal or otherwise as to why one way is better than another?
For reference, I plan to test the technique on a CDA. So color is crucial, as is reducing the amount of roast flavor. I plan to use Carafa II or chocolate or both, since it's a leftovers recipe and that's what I have.

I've done a couple of Black American Ales (heh) what I'll say is Carafa *Special* III is your friend (note the special it's dehusked). When I've done them I've done one of two things. The first option was to add the cold extract during the final 15 minutes, then immediately knock down to do my whirlpool at 170, the second option was to add at flame out, do a 20-30 minute hot whirlpool, then chill. I like both methods, and they both ensure that any bacteria in my cold extract are nice and dead.