The top person works the least. He has to be fresh. Most doctors can't directly answer the question of how a presidential campaign might affect someone who has heart disease. These days, as long as pilots pass various stress tests, some are allowed to fly again even after surgery. It depends on the individual and how they deal with stress.
Some stress is good stress.
If you love what you are doing, it can be a motivator. If you hate it, that's a totally different kind of stress. Regardless of how well Cheney deals with his stress, it's highly unlikely the public would hear about any serious health problems even if they did occur. Though it isn't always rational, human beings associate health inextricably with goodness and strength. The image a powerful leader must have is of good health.