Dips are more pre-disposed to producing dysfunctional mechanics than most upper body exercises. When performed incorrectly, dips will wreak more havoc on the shoulders than just about any other movement.
However, when executed with perfect technique and appropriate body adjustments, they're actually therapeutic on the shoulders. It's important to master the correct form in order to reap the worthwhile benefits while avoiding the potential drawbacks.
The best cue is to keep your shoulder blades back and down. The first few reps show the incorrect position. In the last few reps, the shoulder are back and down.