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, that is the final answer.
Some dances are sacred, and are never to be photographed.
If you see a dancer that you would like to take a picture of ASK THEM first please remember 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. Be wise, and be cautious - ask first. Do Not Ever Touch A Dancer's Regalia without permission.