No hair must be cut, it is one of the four major sins in Sikhism requiring re- baptism (that is if they were baptised in the first place). Hair is considered a gift from God and an acceptance of his will. It is also important to spirituality - You'd have to go really deep into Sikh philosophy to understand exactly how, but it plays a big part in spiritual progression. Finally it, along with the turban is a symbol of the Guru (Guru Nanak-Guru Granth Sahib Ji- the 11 Guru's who revealed Sikhism) and shows pride in the path, and not being scared of letting the world know you who you are and what you believe (this ties in greatly with the religious opression Sikhism was born under the Mughal Empire in India).