You will find people that say "the bar of slate should only be on the back". Now; how much sense does that honestly make. You have that feature in your flock or you don't. I've yet to see a exhibition judge ever check a thigh or breast for undercolor. Regardless of what a person says, undercolor does indeed contribute to the overall color depth of this breed.
It's not just people, it's the Standard for the breed. You can either breed to the Standard, or not, no one is forcing anyone to do so. But judges can and do notice bar of slate where it's not supposed to be, and some will knock you for it.
Just fyi...