When you free range chickens you need to worry about predators. Like neighbors cats and dogs, hawks, coyote, fox, etc. There is always a certain risk factor involved in free ranging your birds.
I let the chooks roam the back yard, but not without a fence. I guess it just depends on where you live and what kind of predators you have. There aren't too many here, so that's not much of a problem for me.
Yes you can let them roam the yard, but just don't let them cross the road. There are just too many questions asked after the chicken crosses the road.
As other have said the chickens run a risk from preditors even in a yard. They can also fly out of the yard. So it is up to you and how much you are willing to risk.