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Magnetic variation can be pretty dramatic, especially at latitudes past the tropics. It's reasonable to use a combination of magnetic compass with dead reckoning.

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And ever changing - drift is quite visible over a decade or so, and using old maps can cause real problems if you’re only using magnetic when you get even as far north as Seattle.

And this is why when doing any sort of navigation activity you should always check the publish dates on your maps/charts and act accordingly!

My uncle was a private pilot. He used to take me flying from time to time.

I remember him saying that it's legal to fly without a map, but it's not legal to fly with an outdated map. If you have a map, it must be up-to-date.

Don't some maps actually show the current rate of change? I seem to remember some of the USGS maps I purchased showed this.

You're right that they do often show the local magnetic declination's rate of change (around the time of publishing), but even that goes out of date eventually. For relatively short periods (maybe on the order of years or possibly even a decade or two, depending on the particular location), applying this rate of change may give a good approximation of the present day's magnetic declination value, but beyond that the errors can become unacceptably large.

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