Ive never washed my DMC thread before I use it. Its a pretty colourfast thread. However when purchasing brands you are not familiar with or in kits, I would wash them as Ive had other brands run after the stitching was done.
Beware of reds, purples and blues, these are most likely to run. If I am not sure of the quality I will swap them out with a close shade of DMC and donate those unsure threads to charity.
Ive never pre-washed any DMC, but I found out that Polish brand (Ariadna) - red, blue - better be pre-washed. DMC is a fine brand and Ive never had any problems with it :)
I havent prewashed DMC or any specialty threads. I am just very careful with my projects (clean hands, etc.) so that I dont have to wash them after I finish them.
In another forum I found a test done by a stitcher: she put a small sample embroidered with DMC in bleach and color resisted very well (it was blue, I dont remember the number), so I think DMC quality is very good and I dontt think I need pre-wash thread.