Or being unafraid to offend means they are edgy and bold. All in your interpretation.
As for the term "off the reservation", whether you find something offensive or not all comes from your perspective. I choose to not let things offend unless it is obviously meant
as an offense. I don't think Dean meant it in an offensive manner when he said it, just like him saying Siamese Twins wasn't offensive in intent. Sam is more politically correct, which being an educated man he is more in tune with that. Dean is the common man and as such he speaks his mind with colorful phrases that he sees nothing wrong in. Hence, I don't either.
Living in AZ surrounded by reservations and knowing lots of Native Americans, I don't think most would find the term offensive. I dare say they would say it themselves and wouldn't blink if someone said it to them. It's not like anyone in this day and age believes that 'Indians only belong on the rez'. I think we've evolved a long way from that. And their sense of humor about past historical
deeds is pretty good, all things considered. I went to see a comedy duo who specialize in their Native American sensibilities and I honestly was shocked at some of the comments that came out of their mouths, but the Native Americans in the audience were roaring with laughter...seeing the truth in what they were spoofing.
I hope Supernatural doesn't lose viewers because of offending them with their views and digs, but then if we do...they probably weren't the sort of viewer to accept the rest of Supernatural either. Our show isn't meek and mild or for those without a stout heart and strong immune system.