My cleaner shrimp has been in my tank years and NEVER cleaned a fish. I think my tang is too skittish. When i feed, i can get the shrimp to swim up and eat out of my hand and it will start to clean under my fingernails and stuff lol...
In another members tank I saw an angel let the shrimp clean its outside gills AND let the shrimp walk in its mouth to clean the gill from the inside. When the shrimp was done, it just walked back out of the fishes mouth.