Not sure where mine is set but I assume it’s true north. In the grand scheme of things it probably won’t matter. Mainly because you are using it for reference on a map, not for true navigation.
By navigation I mean hundreds of miles over open water.
Wow haven’t heard those terms since army land navigation course!
I think biggill is right, true north should work fine as long as you’re not travelling long distances. Magnetic north should only affect compass bearings.
You would want to leave most GPS devices set for true north. When you look at the map on a GPS device, the top or up is true north, so most of the time you’ll want your direction on the compass to align precisely with the directions you see on the map.
The main reason you’d want to set your device to magnetic north is so you can use the compass reading from the GPS to navigate with using a magnetic compass without calculating declination.
For example, if I’m out on Lake Superior and I want to head for the south tip of Isle Royale, which is currently over the horizon and not visible to me, my GPS and mapping software, set to true north, may tell me to take a heading of 080.
By using the “magnetic north” setting, I would get a heading that I can use directly on the marine compass (which of course gives only matnetic headings) on my boat’s dash to steer exactly that heading I need to go which might be something like 082.
If I had the GPS device set to “true north”, I would have to do the old-fashioned declination calculation to “translate” between the true north of the map/GPS and the magnetic north of the compass on my dash so that I knew what magnetic compass heading to steer.
So the setting helps eliminate the need to covert true north readings to magnetic north readings that are usable with a conventional magnetic compass. This can sometimes be a significant difference, I have seen aviation charts where a correction of more than 10 degrees is sometimes required.
Grouse
way back when I took my pilots training course,we were shown that true north and magnetic north are a couple of hundred miles apart and it can vary quite a bit from year to year.
if you are using it for navigation over long distances,keep it on true north as you will end up many miles from your destination.
if the fish arent biting and you are bored,set up a destination point.
try navigating to it with the true north setting and then do it again with the magnetic north setting from the same starting point,you will find that even in a short distance,( less than two miles ) it can be off by more than you would think
edit to add: like Grouse said,it will depend on what compass heading you are working with as to what choice you make as far as magnetic north or true north over extended distances,ie in a boat.
It doesn’t amount to much difference. You can use a calculator like this one:
https://www.ngdc.noaa.gov/geomag-web/#declination
SR
It doesn’t amount to much difference. You can use a calculator like this one:
https://www.ngdc.noaa.gov/geomag-web/#declination
SR
OK, I read the first paragraph. Was that in English because my head is still spinning. 
It doesn’t amount to much difference.
This is true IF you are in the Midwest. Fortunately for us here in Iowa/Minnesota the difference is only about 19 minutes of a degree. Going further West in the US, can get you a 16 degree difference, which will get you lost in a heart beat.
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