fat down will definetely give you more (and better) bark. with the fat up, it does baste, but one of the results is that the "basting" goes to the bottom of the roast, making what would be bark stays a little "wet" preventing it from becoming a good bark. for selling, I do the fat up, because I end up with more shredded and less bark, but still enough for those that want it. For home, I do fat down so I have more bark!!!
It really is a preference thing.