Must be pretty common behavior for them to do this nibbling as there are many posts online about it: (Examples)
Why Does My Dog Nibble With His Front Teeth? | eHow.com and
Why does my dog "nibble" us like he is eating corn on the cob? They call it flea biting, corn-on-the-cob biting, corn nibbling & blanket nibbling. Causes are ranging from grooming to play to comforting to boredom.
Redirecting seems to be the best advice to try to stop the behavior. Give them something you want them to chew on instead.