The goal of the class hierarchy is to allow you to organize objects into a relationship which allows a particular object to inherit the code of its ancestors. Once you have identified an effective organization of types, you should find that a particular technique need only be implemented once, then inherited by the children below. This keeps your code smaller, and allows you to fix a bug in a particular algorithm in only once place—then have all users of it just inherit the fix.
You will find your decisions for adding objects change as you gain experience. As you become more familiar with the existing set of objects and messages, your selections will increasingly “fit in” with the existing ones. But even a Smalltalk pro stops and thinks carefully at this stage, so don’t be daunted if your first choices seem difficult and error-prone.