I don't butcher my birds in front of the flock but its not intentional, just the set up of our home/yard. However, all birds destined for freezer camp can see what's going on. I put all of my dinner guests in a dog crate next to my cone, and they don't seem to worry until I actually hang them (that initial complaint about being upside down- but then they settle).
Last time, I think the left overs in my coop were stressed, but I wonder a bit if its because they can hear the comotion but can't see what's going on... maybe its actually WORSE for them if they can't see it! They may also have been stressed because their breakfast was delayed due to processing. Who knows?
With that said, we often butcher steers in front of their comrades and they don't worry either. Temple Grandin has some excellent research about reducing stress at slaughter- that blood smell, even in mammals, does not cause stress, but that things like excessive noise, visual stimulus, and aggressive handling can significantly effect the welfare of the animal. So at home, I try to keep my waiting birds in a comfortable, quiet, cool/dark environment while they are waiting to reduce their stress and handle them as gently as possible. I think that goes a lot further to help reduce stress than whether or not their friends can see them.