This is never an easy thing to think about. Long story short, my beloved rescued Airedale contracted a fatal blood disease. He had been at the Vet for several days on another issue. When they discovered the blood virus, they called and told me that there is very little chance for survival and that putting him down would probably be the best option. So I left home to be with him while they administered the drugs. He died during the 10 minute drive it took me to get there. I sat with him and cried my eyes out for 20 minutes before my mom showed up and got me out of there. I've never forgiven myself for not being there with him. I will not, and cannot bring myself to ever have another Airedale.
Conversely, my wife's pup of 17 years had gotten to the point where he could hardly walk or poop because his back legs were so weak. We decided to have him put down. Of course, in the couple of days before we were scheduled to do it, it seemed he got a bit better. But we stayed the course, knowing that it was best for him. We took him in and held him. Once the drugs were administered and he began to fade, my wife collapsed in hysterics crying and apologizing to him for what we were doing. It kills me to think about it even now, and this was several years ago.
Be there. Don't be there.. Doesn't matter.. It sucks either way.
Sorry to be a downer...