Personal decision.Would be based on the pt's condition, distance to the nearest trauma centre, weather, extrication time, the golden hour.
If you have a serious trauma pt trapped in an vehicle, chances are you might want to have a chopper ready to get him to the horsepital to make it that "hour". There are so many things that can affect your decision. I have only flown one patient. He was presenting with your classic MI signs/symptoms, unstable vitals and we were over an hour away to closest ALS backup as well as medical facility that could actually handle this particular case.
Made the decision on the simple fact, to me he was very ill and we were away from everything (like always). I did second guess myself. Later learned it had been the right decision.
It all depends on the situation. I've worked in a air medical dispatch centre and know the list of your typical reasons for putting a chopper on pre-alert or launch them, but having said that I know I second guessed that day.
Situation dependant. I don't know it would be possible to make a solid yes or no list as to when you should or shouldn't send out a chopper. Just keep in mind that if you're flying your pt based more on you wanting a thrill, you're possibly putting someone's else's life at risk who might have actually needed that chopper. Use common sense.