Some large Pow Wows require that you register your camera, and will put a number on it. The announcer will tell the audience when pictures are permitted. I have seen cameras confiscated and film destroyed because people did not listen, and did not take instructions seriously. If there is a particular dance you want to photograph, Ask Permission Of The Announcer. If he says no, give it up.
Some dances are sacred, and are never to be photographed.
If you see a dancer who is especially striking, ask if you may take a picture After He Or She Leaves The Arena. It would be a courtesy to take his or her name and address and send a print of the photo. Human kindness and consideration are always long remembered, and deeply appreciated. Protocol for pictures is different depending upon the part of the country you're in, and the customs of the Pow Wow Committee. If you see videos being taken, and professional looking camera crews, chances are they represent a Native organization which sells Pow Wow videos, or newspaper and magazine photographers who have gotten permission and clearance before the event.