Yup, uphill has the right of way... But, looking ahead and being courteous goes a LONG way on narrow trails. Always be on the lookout for a place to pull over. Quite often I've seen both parties pull over to see what the other will do.
I've heard that the uphill right of way was also instated before fuel pumps were a regular item on cars. Fuel tanks were mounted higher than the engine, and relied on gravity. If the slope was too steep, fuel would stop flowing and the engine would die when the bowl emptied.