A series resistor would be more "safe" in case 5V isn't really 5V but gets shorted to some other external voltage, like 12V. In this case the series resistor will limit the current going into the pin and might save your AVR from an untimely death.
Since it is a high-impedance input you can add a fair amount of resistance (1k or so) and not really change the behavior much. Just make sure that any pullup/down resistors are at least 10x this much (as a guideline) so you still get nice clean high and low voltages (the series resistor will form a voltage divider with the pullup/down).
And as others have mentioned, connecting the switch to GND and using the built-in pullup is more common.
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