I believe that the lack of certain vitamins or other things the body needs can cause gray hair. It wouldn't surprise me if stress had negative effects on the bodies ability to absorb vitamins or caused the body to use up vitamins etc in larger quantities. In general stress does have a negative impact on health.
However, most people get gray hair with age and it would be difficult to differentiate people going gray from stress and people going gray with age unless there are some people who go gray and then gain back their color after the stress is removed.
My father and my siblings suffer from Vitamin B12 deficiency (clearly a genetic condition). My father had snow white hair from his twenties and a doctor told him that it might have been caused by Vitamin B12 deficiency. I and my siblings are all starting to go gray in our thirties. But it is quite possible that the two have no connections.