Happens to me too, especially when you get to the heavier weights. I'm doing 4x15 with 95lbs and my left pinky is usually feeling pretty beat up. If you are at a gym, i'd try the different bars and grips they have available to find one more comfortable. Since it's an ab exercise, your grip is mostly irrelevant.
Since I'm doing these at home, I generally save them for the end of my workout and suck it up. If I lose my grip I get 'angry' and bang out that last few reps with a little more venom.
If it got to the point where I was approaching injury I'd find another ab exercise to do..