For what it's worth, I agree 100% with what Charles said. In college I worked on creating a HUGE ESD database for ASIC chips in a small chiphouse in Idaho. We had to of coarse qualify Military spec parts to certain MM, HBM, & CDM (Models). blah blah..

While working in the potatoe harvesting (manufacturing equipment) industry as an intern I say my design engineer (boss) place protection to all inputs on our control boards but almost never to outputs for ESD reasons. We commonly placed a resistor, zener, cap network on all our inputs for just one reason....to IDIOT proof our boards against the assembly line. We had to gaurantee that no matter if 12V or GND -12 was connected to any input or output we would not loose a board or a PIC.

What am I getting at???

The PIC have ESD BUFFERS BUILT IN THEM, that's right all pins, have some kind of buffering (double diodes for example) on the inputs to protect them from ESD events. I am not sure to what levels (ex: HBM 5000V MM 2000V ?) If someone know what JEDEC standard they pass let me know I have not looked....I guess I should now that I am getting all preachy).

Yes you can build up static on machines, hands and touching a pcb with a pic just right could burn up a port if the ESD is high enough, I would implement the 330 resistors on outputs and zeners on Vcc mentioned below if worried that much about ESD.

I did nothing but eat, drink, sleep, think ESD for about 7 months as an intern and have not worried about it all that much with PIC's since then. Probably just a bad taste in my mouth or confidence in the designers who make PIC cores.

stepping off soapbox now.