I've only ever seen one of mine bite and that was the one who only bit his fins after he got sick with a mystery illness that I ended up not being able to cure. He was a rosetail, and up until that point never touched his fins.
Pinkerton might have bit due to his coloring, he was a pale pink butterfly DT, and from what I hear fish with his coloring seem to bite more then others and some suspect there's a genetic component to it. He'd stop for a week or so then start up again.
Bruiser, royal blue VT, only started biting after he ripped his caudal and dorsal fins off on the filter intake tube. He let the dorsal grow back but insisted on biting back the caudal ever time it got around half grown back. I almost suspect that he liked it that length. For reasons that I can't figure out, after months of his trimming it he stopped and has left it alone for around 6 months now.
Cinnabar was a red VT and never touched his fins. Lagniappe is a yellow VT and leaves his fins alone. Picasso is a marble rosetail / double tail and he's never bitten his fins. Cork is a blue, red, bicolor HM double tail and he leaves his fins alone. They are all in heavily planted tanks, but the tanks vary in size from 2.5 gal to 20 gal, they are all fed the same diet.
I sometimes think it's luck of the draw whether or not they bite. It does seem that the longer the fins the more chance that they might but that's probably because of ease of access to the fins.