That shoulder-width recommendation is just a starting point and a guideline. Do you feel too cramped with your current bars? If so, go wider, if not, don't bother. As bradtx noted, the width is specified at the ends of the drops and can be specified as center-to-center (c-c) or outside-to-outside (o-o) depending on the maker.

Most catalogs that offer various makes of handlebars will say if the listed width is as c-c or o-o. As a general rule the o-o measurement will be 2 cm larger than the c-c measurement on any given bar so one maker's 42 cm c-c bar will be the same width as another makers 44 cm bar specified o-o.