We don't start inter-service pissing matches, but we make it clear that we're neutral in terms of the crime and only there for the health and safety of the victims. If the cop has an EMT #.. They can recall us, or if they are willing to take responsibility for the life of the victim, they can. However if there is any reason to second guess their decision, we continue in, and ask that the in-charge officer sign something.
If it's a situation where no services are needed, at all, a non-reportable crash, we'd cancel. Either way, we rolled/stood by/canceled, still have paper work to do.