No it doesn't need to be included, it was introduced as a tracking thing for their labels years ago and seems to have no relevance, if you are handwriting or printing labels off.
I will caveat on this though: If you are sending to a collection point eg an Argos, or the item is going to a third party freight forwarder for onwards travel then you MUST include any reference number shown on the address as this is the unique identifier for that buyer.
