Sitbone Width and Saddle Width

Don't go for a saddle narrower than your bones, you'll be resting on soft tissue and pelvic floor muscles rather than on the bones. (exceptions for tri saddles and extreme aero saddles, of course, where you are more on the pubic rami than the ischial tuberosities)

General rule of thumb: get a saddle with 1-2 cm wiggle room to either side of the sitbones. As you shift positions, climb and descend, turn, etc., you will be moving your pelvis relative to the saddle. Also, if your bones are the exact width of the saddle, you will be perched on the frame of the saddle rather than on the nicely engineered "sit spots" of the saddle.