General wisdom (and often testing) infer that the middle-range of apertures gives the best edge-to-edge sharpness with most lenses. While that may have been the foundation for the person's recommendation, it's completely specious for portraiture, and even for some landscapes. After all, the trade-off is with shutter speed and film speed, and there are simply too many situations in which those factors may be more important than aperture.
Shoot the images the way you want. After all, it's your film, your lens, and your camera, and you want the images to turn out to your specifications rather than those of another "expert." The person who made the recommendation should be satisfied with his own equipment and quit messing with yours.