First, why should you clean your roof valleys of leaves and debris? Roof valleys are made with shingles or sometimes galvanized metal. These materials will last longer if you removing sitting debris that holds moisture. Second, the valleys will simply look better and cleaner.
If you can see and reach the leaves in your roof valley, then the Gutter Viper will clean them. It'll be easier if the debris is dry. Even houses with gutter guards can accumulate debris in their roof valleys and over their gutter guards.