It is a good safety practice to use one yes... But, It's not going to stop you getting killed if you touch BOTH conductors. Or if there is a fault and you touch the case and one conductor.
You have to remember you are still dealing with lethal levels of AC. With a system using an isolating transformer you have no earth to protect the case and supply cable. So, if there is a fault and the case becomes live there is no earth path to leak any current and trip any breaker.
Generally it is said that you should remove the supply before doing work. However, this cannot always be done. In these cases BE BLOODY CAREFUL.
Larry