Testing nulls and exceptional cases is always important because developers often forget to handle such cases in their code. But remember that you do not want to write tests that never catch a bug. Before writing such tests, you should understand the overall picture of the software system (and its architecture). The architecture may ensure that the pre-conditions of the method are satisfied before calling it.
If the piece of code you are testing is very close to the UI, exercise more corner cases such as null, empty strings, uncommon integer values, and so on. If the code is far from the UI and you are sure the data is sanitized before it reaches the component under test, you may be able to skip such tests. Context is king. Only write tests that will eventually catch a bug.
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