Most people seem to have got the message that if you want a nice green healthy lawn you must feed it from time to time. The same also applies to hedges.They languish for decades without a decent feed – getting thinner and thinner until they finally succumb to disease.
A healthy well-fed hedge is likely to be more resistant to all the various lurgies that can afflict them: box blight, shothole, honey fungus. The same applies to topiary.
The rule is as for shrubs or anything else: if you prune or clip, you must feed.