In general for reliable operation of any digital IC you need a bypass or decoupling capacitor as close to its power pin as possible. If the IC has more than one power pin then you need more than one capacitor. Atmel recommends one for the AVcc pin as well.
The Aref pin had to wire a .1ufd bypass cap from that pin to ground for a slight noise reduction improvement for AnalogReads.
These are some info what other had done.
ISP Shield for Arduino