I always rode 42s because that's what came on my bikes. I didn't think about because by conventional wisdom, my shoulder width also dictated 42.

When ordering a set of bars to try, I ordered 42, not realizing that this particular model was tapered. They felt weird, but nice when I first rode them. I measured, and found they were 39 across the hoods. I didn't like those bars for other reasons, but I've replaced all the bars on my bikes with 38s. I like those a lot. I'm not sure why, but I'm a lot more relaxed through the shoulders on them.