It is a very good idea to do so.

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Unscrupulous folk will resell your old plates if you left them and a few years later you'll start getting notices about fines and penalties in places you've never driven in in your life.

Ask me how I know.

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"Turn in" might be the law technically, but apparently it's not really enforced, so at least keep the plates. Just don't leave them on your vehicle, whether you're selling it drivable or selling to a junkyard after getting totaled.