I actually don't think it is. From what I have heard it seems that the only major advantage would be af am, then followed by hispanic, but they don't always get a huge advantage that much, and then native american, I think that that is just a box on there to check because they want to know how many go to their school.
To the OP:Even if you weren't a native american, your chances would be very good to get into a top 10 (certainly top 14) school. As a strong native american applicant, you will likely get into all the top 10 schools, except for maybe yale, but only because it's so tough to get in there regardless of race (but you do have a strong shot there). Now as for preference of NA students over black/hispanic:Contrary to what logic might bring us to believe, native americans ARE NOT highly sought after by adcomms from the top schools. In fact, the AA applicant most sought after are black males because they are so grossly underrepresented in all higher education fields and because there is such a premium placed on their advancement (not to say that NA's aren't in a similarly dire situation, but their plight is not nearly as politicized or focused on by the media/politicians as that of African Americans and Hispanics (to a lesser extent).