I think all of us have to maintain our own sense of balance in our work. Not everyone is cut out to be a James Nachtway or a Eugene Richards, and even those guys have to take a time out every once in a while to decompress. I know in my own work, I have developed over the years, certain comfort levels that I will not cross. It is true that the camera can be a great insulator from the reality happening in front of you. I just try never to lose sight of the reasons why I do what I do and always be aware of the line between telling a story and exploitation.
Just being able to desensitize oneself from the tragedy in front of the camera is only part of it. Being able to have compassion and empathy for your subjects is also necessary to make really powerful images. As Les pointed out earlier in this thread; sometimes we make images of difficult subjects in an effort to help where we might not otherwise be able to. One of the most rewarding things about my job, is being able to glimpse into the lives of others and tell their stories through my photographs.
I covered several stories in the aftermath of Katrina and all of them were focused on not just the tragedy, but also the compassion and selflessness of the people that were there to help. The photograph that still haunts me is of a man looking out the window of a an Air Force C-130 as we were taking off from New Orleans on a medical evacuation flight. As I composed the shot, I remember thinking "here is a guy who through a situation completely out of his control is now looking at his home for the last time". Picking up the camera and sticking it in someone's face in a time of tragedy is never easy but knowing that his story may have an effect on others makes it all worthwhile. Tim, if you do get a chance to back to your sister's clinic, do it. I think you'll be glad you did, and you may find that it's also nice for the people there to know that someone recognizes their effort enough to tell their story.
It is also interesting to note that along with the DSLR's hanging off my shoulders as we got off the plane, there was also a Rolleiflex 2.8F loaded with Tri-X hanging around my neck. Some of my favorite images from that story were from the Rollei. It was also funny to see that little gleam in the eye of other photogs as they noticed the Rollei.