Dietitians have one of the least stressful careers out there. However, they do occasionally have to deal with intense situations. Anyone who just found out that they have a disease and need to change their eating habits isn't going to be especially pleased with the dietitian telling them such things.
However, unlike other careers in the medical field, you won't have to be "on call" or work late hours. Most dietitians work normal work hours unless they have someone call in for an emergency donut intervention at 3:00AM.
Those who are self-employed have the added stress of dealing with insurance companies and building their business. A lot of the stress goes into building a client base that helps you keep your head above water. If you're looking for a career that doesn't have a lot of overtime and helps people, this one could be for you.