Protecting your computer from stupid (arduino) mistakes
Both the arduino and a USB port do have some protection against wrong connections and such. The arduino has a diode to prevent wrong polarity to reach your power supply, it also have a resetable fuse (the kinda big, often green SMD part close by your USB connection on your Arduino) that should protect your USB port pretty good from its side of the cable. Your USB port also have some good protections going on as far as i know, so basically it should be hard to break a USB port by accident.

That was the theoretical part, In real life things aren't always what they claim to be, so I use a cheap powered USB hub, that i always use as a first line of defense.