To what extent does Casy's failure to say "Amen" at the end of his prayer foreshadow the need for the Joad family to adjust or abandon their adherence to rules and artificially imposed doctrines as they embark on their journey west?

Casey is always reluctant to reclaim his position as a holy man. Casey has had a falling out with God. It would deem that God had not comet through for these people and blind faith wasn't going to help them. I think Casey felt that saying "amen" might give the Joads a sense of false hope in the divine, a divine which he no longer believes in.