My supervisor was very helpful and instinctively understanding. He was at the venue of the viva to greet the examiners, then to greet me as the examiners conferred in private. He took me to one side and reassured me that it would be OK, then left me alone to collect my thoughts. He was then on hand to hear the verdict, but never once expressed an interest in being present.
If he had asked, I'd have politely refused. Enough pressure to perform already, but at least the examiners are strangers so you can put up your academic persona. The supervisor knows you so well this would be very hard.