You should make sure that you have a confirmed ticket before you board any train. It is considered a criminal offence to travel without a ticket or even with an unconfirmed ticket. We strongly recommend you DO NOT board a train if your ticket is not confirmed. However, in case of an emergency, you can go to the station master and get permission from him to travel without a confirmed ticket.
Just for your information, you cannot board trains like Shatabdi, Rajdhani, Duronto, and Garib Rath with a waitlisted ticket. You must have a confirmed ticket to board any of these trains.