I think it's a very personal choice.
I have friends who did CIO at a very young age and so they didn't have to listen to the wailing went to their basement and watched a loud movie. They swear their method is best.
We have tried several of the CIO methods with our three kids. One was a "puker" when she got upset. The ped called it a "sensitive gag reflex", so letting her go with no assistance wasn't an option unless we wanted to end up having to shower her, change the sheets, etc. every time. For her, we'd go in, NOT talk and just shush her, less and less each time, until we could stand in the doorway and do it.
The second child was a binky baby. However she would not put it back in herself. So for that child, I had to go "cold turkey" and just let her fuss. It made for a very cranky toddler for several weeks.
Our third baby does some CIO and gets some help. Just when he's sleeping great he gets sick or gets his nap schedule screwed up, we're back to up at nights, sometimes several times. If I know our baby doesn't need to be fed, I try to let him CIO. However if he starts to get "hysterical", I will go in, pick him up, and rock him until he's 1/2 asleep. It depends on HIM as to if I let him CIO by himself or go and "help" him. Sometimes he really doesn't calm down until I feed him...then he just passes out.
I'm sorry I don't have a cut and dry answer. I think CIO depends on the parent's tolerance to listen to the crying and the child's reaction.