Answer:

The reason you want to keep the shoulder girdle down is that if you let it come up, the trapezius muscles will assist with the press, reducing the tension you place on the delts. To better isolate the delts, you need to minimize trap involvement and this means keeping the shoulders down.

Raising the shoulder girdle isn't the end of the world but you'll get more out of the exercise if you keep the shoulder girdle down.

Here are some more tips on how to perform dumbbell shoulder press and barbell shoulder press.