My personal experience with darts is that they seem to make more of a difference with untucked shirts than shirts which are tucked in. I have two dress shirts with almost identical measurements, except that one achieves a slim waist via darts and the other without (only tailored at the side seams basically). Tucked in, they look identical, but untucked the shape is much different. The darted shirt conforms to the shape of my body quite well. The plain one curves in a lot at the waist if viewed from the front, and billows out in the lower back if viewed from the side. It is as if the darting causes the shirt's fabric to distribute evenly around my waist, and the lack of darts with a slim waist causes the fabric to gather unevenly in the lower back. I don't know enough about shirt construction to speculate as to the cause, but among all the shirts I've got, this behavior remains consistent.