AREF on atmega328 if analog is not used
The AREF pin has no required connection. 

If an external reference is needed, it can be connected to the AREF pin, but then care has to be taken to not connect AREF to an internal voltage source.

If an internal reference is used, the "best practice" is to externally decouple the reference by connecting a small capacitor between AREF and ground, but this is not an absolute requirement and you will usually "get away" with not connecting it.

If the ADC (analog pins) is not used, you can safely leave this pin unconnected.