I was certified as an EMT and spent some time volunteering in the ER, and while I did see some things that were somewhat traumatizing when I was young, I don't think I'm worse off because of them. Seeing someone my age die hurt. But it also made me appreciate the fragility of life a little more and turned me into a more careful driver (which was a good thing for 20 or so year old me). That said, it was only a day or two a week for a year or two, so much less exposure than someone full time gets. But I'm really glad I did it.

(But, y'know, this is a very ymmv kind of thing. I enjoyed cadaver lab and thought attending an autopsy was fascinating; other people had very different reactions, etc. no idea what causes us to differ there, but differ we do!)