Having a very minor episode of adrenaline dump last month when I rear ended someone, I can say that the time spent reading about and rehearsing what to say was well spent.
After making sure my victim was okay, while waiting for the police to arrive, I went over in my mind what to say. I did not want appear uncooperative, and still not to admit to anything more than the basic, obvious facts.
This wasn't a shooting, I realize, but on the other hand it was a training experience. Even something that minor the adrenaline flows and you have to work hard to think clearly and not babble on. I can't begin to imagine how a real, major event would effect thinking.
Point being, having given prior thought and practice as to what to say and not say, was invaluable. I got a simple ticket and a ho-hum for the officer. The victim retained an ambulance chaser who refused to take the case, BECAUSE, there was no record of anything but the simple facts. Nothing to hook their teeth into. The same would go for a grand jury or DA.