The most recent 3rd Edition of the Backyard Astronomers Guide by Terence Dickinson and Alan Dyer has greatly added to their DSLR astrophotography section of past issues. In this book the claim is that they have done it both ways for quite a few runs and feel the in-camera method is best (for quality, not for productivity) because the darks are taken when the chip temp is as close as possible to actual chip temp.
The book claims that darks for a fully cooled CCD camera have no problems with setting up a little fridge at home even and take darks at home which is the ultimate time saver for the field. Warning: Wife alert if you try this in the normal fridge and forget to put temp back!
As far as taking darks I have found that I must take darks for 4 minute or so exposures as they are the only thing that will take out tiny single pixel blue or green or red dots for my camera which starts to show these dots at such long exposures. These dots are even more of an issue when stacking or bringing out detail with certain post processing steps.
My experience in Astrophotography is very very minimal (mostly have been a visual guy) so I cannot offer my own opinion beyond the pixel issue. That is why I mention Terence Dickinson book as he is one of the key contributors to astronomy and I respect his authority due to reputation.
The Backyard Astronomers Guide is fantastic book/reference for many other reasons as well. ISBN -13:978-1-55407-344-3. It is a bit expensive but if one is into astrophotography, one is already numb to the cost of many things.