I never have until yesterday. I figure there is nothing I can do with no equipment and EMS will be there immediately anyway, etc. I stopped yesterday because I was in the middle of the boonies and saw the crash and it looked bad. I figured it would be a while before EMS got there. I didn't even have a signal on my phone.
It was a bad wreck. The only thing I could do without risking injury to myself was hold C-spine on one person and yell instructions to people. Plus I busted out all my ER nurse charm ("let's take care of you first, I'm just going to hold your head, I'll stay until the ambulance comes..."). Which all told is a lot better than nothing. But I am sore all over today just from my muscles clenching up and from holding this person's head.
I'm sure I didn't change a whole lot. Was it worth it? I don't know.