I just went through this myself. I went to a dealer, and sat on their assometer, and my sit bones are 135mm apart, center to center. I chose the Romin Evo Comp in the 168mm width. If you look at this saddle, the outer edges are turned down, meaning the outer edges would provide no support to your sit bones. The reason I chose the 168mm wide version is because the two high points my sit bones were going to end up sitting on were about 135mm apart on this saddle. Had I chosen a 155mm saddle, my sit bones would not have been resting on the highest points on each side of the saddle.

You do have to take into account the shape of the saddle when decided which width to buy. If a saddle is very flat, all the way out to the edge, you might be able to use a narrower version, but if the saddle is rounded off, you'll need a wider version. I do agree with others - go to a dealer who will take the time to measure and fit you to the proper saddle, and who gives you a guarantee, in case the saddle is not comfortable, despite their best efforts to fit it to you - which often happens.