If you put more than 6V on the +5V pin you could damage the ATmega. Always use a REGULATED 5V supply when providing power to the +5V pin
There was some talk of 5V on the +5V pin possibly damaging the regulator but on reading the regulator specs I think that is only possible when there is a large (1000 mF) capacitor on the 5V line AND the input of the regulator is shorted to ground.