Depends on your hair type and the water you're washing it in. If you have hard water or use products with things that coat/leave residue in your hair, you might benefit from a cleansing shampoo once in a while (re: weekly/bi-weekly) to clean it out.

Most commercial shampoos have silicone based stuff that coats it and makes it "feel" and "look" silky but can be damaging or detrimental over time if not managed/maintained.

Sorting out what works best for your scalp and hair is the slightly easier bit, the water can be a hassle.