In NYS, the fire chief is responsible for determining and reporting the cause of the fire, therefore sometimes they can determine it themselves without a C&O response. I believe they get basic C&O training as part of their command classes. In Westchester, once the C&O teams were formed around 20+ years ago, their use increased over time like any other resource. If my memory serves me correctly, prior to that, the DA's office had one investigator who did the C&O work, obviously he couldn't be at every fire.
As for lights and sirens, I was a PD Detective for 18+ years, I never responded to a scene that already had cops at it (i.e., a past burglary, robbery, homicide) with lights and sirens. There is no need for it on a scene where the active crime is over and the scene is being held down. In my last few years when I was actively investigating fires, I often went out upon the initial call so as to get there early for the reasons stated above. In that case I would use lights and sirens. If I was showing up after the fire was knocked down, I didn't.