My wife and I are still on fairly friendly terms with each other. Plus,we don't want our own problems to cause our children more stress than the divorce already has. We didn't have it written into the divorce decree because we agreed that they would be handed off in a civil manner.
A friend of mine did have it written into the decree because his wife was giving him problems with getting his kids while they were separated.
I think writing into the decree depends on what is needed.