It all depends on what you are trying to accomplish and the amount of air you are moving though the outlet. Are you expecting to get thrust from the outlet? Are you trying to use the airflow to suck the air out of the outlet? Are you trying to minimize drag of the outlet? Or are you trying to minimize drag of the whole object.
Maybe some of these papers may help
http://ntrs.nasa.gov/archive/nasa/ca...1993093623.pdf
http://ntrs.nasa.gov/archive/nasa/ca...1993091791.pdf
http://ntrs.nasa.gov/archive/nasa/ca...1993093645.pdf