I would stop if there was no help there already. I'm sure if there is already an ambulance on scene they would just get annoyed if I tried to help, unless it was obviously out of their control.
I have stopped on some and not at others. It really depends on if my family is with me. I don't stop if they are. I don't want them strcuk by a rubber necker.
If there are no emergency vehicles a)on scene or b)anywhere within hearing/seeing distance, then yes, I will stop. Why? Years back, I was a passenger in a car and we got into a wreck on the Oregon coast. Nobody stopped. That feeling sucked. Nowadays, I'll at least stop and will call 911 if necessary. At the very very least, letting the accident victims know that you're there to help can reduce their anxiety quite a bit. At least that's what I've experienced. Then when the police dept/FD shows up, tell them what's up and then get out of the way. Sure, you run the risk of being called a whacker, but I figure that's better than just leaving someone to suffer on the side of the road or calling 911 when you're already 2 miles down the road.
I will stop most of the time if there are no emergency vehicles on scene, or a cop only. I won't stop if I'm alone at night, but I'll call it in.
If I'm with my fiance, we stop if there are no ambulances, and sometimes we even stop if there are ambulances because combined we know someone in every department.
I generally stop if there are no emergency vehicles on scene and I'm out of my response area. I will stop for any accident in my response area, regardless of what's on scene.
If everyone is ambulatory, I do not stop, even if there is nobody on scene. Depending on where I am at, if I have to turn around to get back to the scene, I might not - simplily practicallity of turning around.
If I am in the county I work for, even if EMS/Police/ or Fire are on scene, and especially if they are not, I am likely to stop and at least see if they need any extra hands. Its the way our SOP's are written. They basically put a "duty to act" on us in our county protocols. In return, if there is any help needed, they pay us automatically for 1 hour.
If I stop, I call 911, give a proffesional scene sizeup. Never do I identify myself as an individual in EMS. The most I have ever had to do was try and calm people down, stop bleeding, and hold c-spine. I dont carry enough wacker gear to anything past that.
I'll stop if I can... but I won't call it in if I don't stop... I have State "EMS" tags and Ambulance Co. license plate.... I don't want to get hit for "failing to act" I'll "not see" the wreck if I can't stop.