I have seen many large gaps in various places in the world. In some areas the only existing maps are very old and were of questionable sources. In the Amazon River basin the maps are mostly reflecting the contours of the tree tops instead of the terrain underneath. There are so many grid systems, projections and datums worldwide. I usually end up right where the lack of good data is. The introduction of GPS has helped tremendously, but you still don't know what the terrain will be like based on the existing maps in some areas.
Speaking of the magnetic declination, it does change. So the map you may have from 30 or 40 years ago will not reflect the current declination. There are places on line to check current declination for your area.
The differences in datums and grid systems is more relevant when you are sending coordinates to someone else. If you are working on your own map you just need to ensure you know what you are working with and that your GPS agrees with your map. There are ways to convert, but it is a pain.
I totally agree with all of Montanero's comments.
Regarding out of date maps, USGS topos for much of Alaska date from the 1950's. For many parts of the state, the first order survey data used to tie into datums was not good. Hence in some areas there are still significant "busts" between adjacent map sheets. Mapping up here is slowly being updated, starting with the populated areas. But it will be a long time, if ever, before Alaska topos are anywhere near as good as in most of the lower 48. (And I know there are still some areas in the 48 states with issues.)
Regarding declination, I strongly second Montanero's comments. Declination changes with time. In some areas it changes quite rapidly. If the info on your map margin is more than a couple of years old, I would check before relying on it. One place to check is at:
NOAA's Magnetic Declination page
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