What's important when training finger strength is not the shape of the hold, but the shape of your hand on the hold. I train on a single campus rung edge, using 3 grips: 3 finger open hand, 4 finger oh, and 4 finger half crimp (no thumb). There's strength enough crossover between these grips to cover most hold types. Next time you train on your board look at the shape of your hand across the different holds, you might find that you're only training your predominant hand position, no matter what the hold.