If you stop washing your hair sebum, natural skin oil, will spread along the hair shaft and coat the hair. On the positive side, sebum is the best conditioner in the world as your sebum is made exclusively for you and will greatly help to keep your hair in good condition. On the negative side, your hair will be, and will look, greasy, and after a short time this oil will oxidise, turn rancid and smell. If you (and your family and friends) do not mind this, ok but these days, with so many excellent products around, there really is no need to go to these measures. It is best to treat your hair and scalp like any other part of your body, wash it regularly with a good quality shampoo and always use a conditioner. You will have beautiful hair ... and smell nice!