It depends on a lot. I think mutations do have a lot to do with it, yes. But I have nothing to show you on hand that proves that.
But no, not all CFers taste salty. I happen to. But my sweat test results were sky-high. Below 40 is negative for CF. 40-60 is borderline. Above 60 is positive.
My sweat test result was 106. Hah. So it only makes sense that I personally taste salty. I actually have to clean out my video game controllers when I'm done playing a game because my sweat gets into the cracks and dries, and leaves a fine salt behind.