Are there any advantages to either DC barrel connector polarity?
Image from: http://www.canadarobotix.com/image/cache/data/products/400/437-1-800x800.jpg



On the figure above, on the left, a connector with no plug inserted. Pins 1 and 2 make contact. On the right, with a plug inserted, pins 1 and 2 makes no contact, and the plug outer ring makes contact with pin number 1.

Some connectors have a third pin that can make contact with one of the others when no cable is inserted and is disconnected when you put a cable in. This is often used to handle dual power (mains brick and battery, for example) or to signal to a processor that a cable is plugged in or no. This third pin is usually connected to the outer ring. If you need to disconnect the positive battery pole, you will have to make your polarity negative-tip. This is one of the reasons I can think of having this kind of polarity plug.