One part is CIO (crying it out), which is crying out of arms when a parent could intervene and pick the baby up. Is that ever OK?
But what about crying in arms? Is it OK to let a baby cry when the baby is held by one parent but you know the baby really wants the other parent? Or if the baby wants something in particular but the baby shouldn't have it?
What if the baby is having trouble sleeping and is just crying? Is it OK to relax and understand that sometimes babies cry when they're tired and not drive yourself crazy over it? Is it only OK in arms, or is it OK out of arms?
What about in the car seat when you're driving?
To me, I wouldn't leave a baby crying out of arms when I could intervene for any reason. I had a baby with colic and she very frequently cried in arms though, and I didn't drive myself crazy over that. I just tried to comfort her while she cried.
I don't like a baby crying in the carseat but it feels like you can't always avoid it and you should just try to keep it from happening any more than necessary.
If my baby wanted me and she were crying in her dad's arms, I would let her cry while being held and loved by her dad if I really needed to do something else, like take a shower, but I would get her if I reasonably could.
So my knee-jerk answer would be "NO no baby should ever cry" but like I said I think the question is more complicated. What are your feelings about crying and babies?