Over the years, I have observed many long time drivers that shoulder check less than student or new drivers. While this may be due to many factors, I want to reemphasize the importance of doing the shoulder check.

Why Shoulder Check?

Because of Blind Spots. Even with properly adjusted mirrors, there are large areas that can't be seen in the mirrors called Blind Spots. The most dangerous of these are to the side, other blind spots are below your field of vision in the front and rear of car.

When do we Shoulder Check?

When changing lanes(mirror, signal, shoulder check)
When turning in an intersection(mirror, signal, shoulder check, scan, shoulder check, execute turn)
In a Cul de Sac(mirror, signal, shoulder check right before start of Cul de Sac, mirror, signal, shoulder check left at start while going around Cul de Sac)
When making 2,3 point turns and U turns(mirror, signal, shoulder check)
When backing up do a 360* shoulder check(mirror, then start by turning body to the left and seeing at left rear and side of car, mirror, then turning body to the right and seeing at right side and rear of car
Before opening door(mirror, shoulder check, open door)
Before pulling out from side of road(mirror, signal, shoulder check)
Before pulling over to the side of the road(mirror, signal, shoulder check)
To turn left or right(mirror, signal, shoulder check)

Just remember, when you plan to change your road position or direction, please shoulder check to make sure that the blind spot on that side is clear!