What is the preferred GRE Writing score for MS/CS student?
There's a reason it's not listed--there is no preferred score, it doesn't really matter.  I am not an expert, but I feel like the writing score is something that can only hurt you, not something that can help you, at least with respect to science or engineering graduate programs.  

More concretely, I can see a very low score preventing you from getting into a program, but I highly doubt the admissions committee saying: "Well, he got a great score on the writing section, that really differentiates him from other applicants; let's admit him." 

For a foreign student or non-native speaker, doing well in the writing section probably carries a little more weight than it does for native speakers; nonetheless, I believe that the above still holds.