BMW recommends using fuel from one of the suppliers based on the list maintained here:
http://www.toptiergas.com/. If you don't, they recommend periodic use of an added cleaner. They don't make a big deal out of this, but I did find it out from a brochure at a BMW dealership's waiting room. Based on the cost of the periodic use of cleaner, any price difference from using one of the recommended suppliers pretty much disappears, if not making them come out cheaper.