With the level of organization required to run 10 and 25 mans, it’s almost certain there will be some kind of official loot policy in place, but the world of 5 mans is more like the Wild Wild West.
So: You’ve succeeded in the first step of a instance crawl: You have found and assembled a team of yourself and 4 likeminded adventurers. If your group is typical, you probably know a couple and you might have had to scour the LFG channel and General in a city to pick up your tank and healer. (Cause isn’t it always shouts of group of 3, looking for tank and healer on your server too?) You enter your target dungeon and commence the decimation of its’ residents. And then a shiny blue dagger drops off the first boss.