Having faced similar situations several times 'professionally' I can say without equivocation there is no one golden rule.
In a recent instance on the side of the road..(yes I'm a do gooder).
I stopped my car so it was between me and the action...Jump out with my mobile to my ear saying in a loud voice.
"Yes...Officer.... the assailants are two white males aged late teens.....Yes I can....take...." I pointed the camera and flashed. They stopped the bashing and ran off.....I and the victim were lucky.
The key is "don't get excited and think"
Back to the topic,
Yes I think it's an excellent idea
It's sad but I believe we have to do these things to give the good a PR boost.