I think we would all be wise to not bind where God hasn't or loose where God has bound. I don't believe it is imperative that the cross is displayed as a sign of faith nor do I believe it is forbidden by God.
The Lord's Supper that we spoken about by Jesus and later by Paul (1Cor. 11), although it is imperative to be observed, does not shy away from the death, blood, and cross of Jesus. Why? Because it is through the death and blood of Christ that we are cleansed and have life (Romans 6:1-8). We drink the fruit of the vine that represents His blood that was poured out and partake of the bread which represents the body that was nailed to the cross.
Also, it should be noted that illiteracy was the norm for the first 1000+ years of the church so images (like the cross) were often used to proclaim truths regarding the gospel.