The fingernails of my left hand, yes, quite often.
NEVER the ones at my right hand. Playing guitar, I need those long, and having had them long since I was something like 7-8, it's like a second nature to take care of them as much as of the essential parts of my body, like eyes, nose, etc. In fact, no matter how hard I try, when I get those nails close to my mouth, they automatically go back, it's amazing how used you can get to something as small as this.
Also, when I break one, I not only can't play properly for a few days, or even around 1 week if I'm unlucky, it all feels strange!! The tips of my fingers are just too used to having that protective thing on top of them.
Playing guitar has had exactly the opposite effect on my left hand - I can never keep my nails more than 1 mm long, and because of the constant and really hard effort I make with them, my tips are so tough that they don't even feel like skin any more, and I can even keep them in boiling water for a few seconds without getting burned (yes, I tested that involuntarily).