In ideal, follow Atmel's guideline for unconnected pins.

13.2.6 Unconnected Pins
If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of the digital inputs are disabled in the deep sleep modes as described above, floating inputs should be avoided to reduce current consumption in all other modes where the digital inputs are enabled (reset, active mode and idle mode).

The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up. In this case, the pull-up will be disabled during reset. If low power consumption during reset is important, it is recommended to use an external pull-up or pull-down.

Connecting unused pins directly to VCC or GND is not recommended, since this may cause excessive currents if
the pin is accidentally configured as an output.

Also:
If the pin is directly connected to GND and you accidentally enable the internal pull-up, you're wasting electricity.
If the pin is directly connected to GND and you accidentally configure it as an output and set it HIGH, you've fried the processor.
If the pin is directly connected to VCC and you accidentally configure it as an output and set it LOW, you've fried the processor.

Aleksei,

Comrade Stalin ( uncle Joe) said

trast iz gud... kantrol iz betr

democratically elected president Ronald Raygun said

trust... BUT verify

so.. what Ihor said