The Atmel processor chips have built-in positive and negative clamping diodes on all the I/O pins that do offer some protection from ESD type events. These do however have limits of how much total energy (joules?) that they can safely dissipate. So I guess it's a matter of how 'bad' a static discharge situation you specifically have to deal with. I would just go ahead and let it be without any special added protection, and only deal with it if there is indeed a problem detected later.
Lefty