Typical Garmin documentation; leaves you in the cold. You could call them or email them and get an answer that might be right or wrong. But the person above who said it doesn't matter was right on if you are only referring to using the 500's routing function to keep from getting lost. Heres' why:
The only time the reference grid matters is if the device is trying to match coordinates with a chart or map. In that case for the matching to make any sense both have to use the same grid reference. Since the chart/map was already constructed using a particular grid reference that means the GPS must use the same.
Just be sure if you are going to construct a route using some application like Mapmyride you match reference systems.
It sounds complicated but a wee bit of practice and you'll have it figured out.