I would say it is perfectly normal and sensible. After all, the Nano is a 5V device. I suppose you could say it is "dangerous" in that if you accidentally connected anything over 5.5V to the 5V pin, damage would very likely occur, the 5V pin cannot take a range of voltages like the Vin pin can. But you could just as easily damage any 5V Arduino by connecting anything over 5.5V to any of its other pins.
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