I feel like you've been misunderstood with all those somewhat shaming answers. If your question is "how to protect myself from wires inside the walls I'm gonna fish or electrical boxes I can't see", it is a very important question. There is not much you can do against defaults already there (like a damaged wire), but as already pointed out protect yourself with your protection equipment, and using a non-metallic fish can help.
I also find that protecting the extremity with electrical tape before fishing can save you lots of trouble, as some fish tapes have sharp edges near the loop. I've seen people skin wires because of this. So, yes, shutting down the power might be a great idea if you're not sure what's behind your wall.