What's right or wrong about calls for "moderation"?

Many things are black and white, but sometimes moderation seems like the right course. For example, you don't want to stuff yourself full of every food that strikes your fancy, nor deny yourself foods that you enjoy. So you should eat moderately. Similarly, you don't want to agree to or deny every favor asked by a friend, but rather do some moderate amount. Is moderation a good guideline in such cases?