We did with DS1 when he was around 11-12 months. His sleep started regressing and he was getting up around 4, so something had to be done. With DS2 we started letting him CIO around 8 months because he was still getting up 1-3x a night when he really didn't need to. He just wanted to comfort nurse and it was getting old really fast!
I think every baby and every situation is different. There is a time and place for CIO, I believe.