Scalp biopsies for hair loss: Is it always needed?

Is a scalp biopsy always needed to determine the cause of an individual's hair loss?

The answer of course is no. A biopsy is only needed if the diagnosis is uncertain. 
In tough hair loss cases, I often like my patients to go a few days without shampooing the scalp. This allows the highest chance of some of the key features of the "suspected" condition to be present on the scalp (and not washed away by a recent shampooing).

Where does one biopsy?

I always biopsy an area which has the most features of the particular hair loss condition I suspect. For scarring alopecias, I often try to biopsy areas that cause the patient's scalp to be itchy, have burning or areas that cause pain.