The shoulders are arguably one of the most important muscle groups for Physique competitors. Nothing else contributes as much to the appearance of a V-taper as a well-developed set of medial (side) deltoids. With that in mind, you might want to stay away from the Smith machine on chest day.
Scientists from the California State University at Long Beach examined the amount of muscle activation that occurs when performing a barbell bench press versus a Smith machine bench press. They found that the pectoralis major, anterior (front) deltoid, and medial deltoid were were all stimulated to a much greater degree by the freeweight bench press. The biggest loser in the Smith machine press was the medial deltoid followed closely by the anterior (front) deltoid. The less experienced the weightlifter, the greater the difference between the muscle activation in the bench press and Smith machine press.