I have hunted and guided waterfowl for many years, and in many differant state. With that said my rule of thumb is if they are doing what you want them to do, don't do anything that might change that. With flagging i only flag when they can see me with one eye. in my region of MN this works, but it does vary from region to region, but a flag is a must! Best way to figure this out is buy a flag (I reccommend avery) and see what the birds in your area like.