RR, in my opinion, the only person who needs to keep a criminal defense lawyer on retainer is a professional criminal. However, it doesn't hurt to know who you're going to hire if you're in trouble, and pay him or her for an hour of their time to explain to you the "mood of the courts" (which is largely the mood of the prosecutor's office) as regards defensive shootings in your particular jurisdiction. That's an entirely different thing from having one on retainer.
While not every case needs an expert, many do. Rules of engagement as generally taught and practiced; speed of action/reaction and similar paradigms (explaining why an attacker might have been hit in the back, for instance), explaining why a suspect might have taken multiple hits and still presented a threat, demonstrating how quickly the defendant might have been killed if he or she had not fired, etc. The only difference between civilians and cops in this regard is that the police generally have more of their own trainers who can be brought in by the defense as material witnesses to explain why the officer was taught to do as he did.
For you, or anyone else who's interested in how the whole expert witness thing works, I can recommend Adam Kasanof's book "How to be an Expert Witness" (Ulpian Press, Arlington, VA, 2006). Adam is an attorney, a retired member of the NYPD, and a gunfight survivor, and he learned at the knee of his father, the late Bob Kasanof, whom I worked with back in the '80s and who was one of NYC's great defense lawyers.