A common trend seen with dips is to alter the angles and mechanics to target the triceps. Unfortunately, altering natural body mechanics is a great way to promote dysfunctional movement patterns and destroy the joints. This is something I refer to as "erroneous muscle targeting" as the lifter has to abandon proper activation patterns in hopes of isolating specific muscles.
A properly performed dip should involve upper body centration with equal stress across the joints and involved musculature rather than isolating a specific area. Besides saving your joints, this places the lifter into the strongest position to handle the most weight for the most reps, ultimately maximizing muscle growth.
If the goal is to target the triceps, there's a more efficient method. Rather than butchering your body's natural mechanics, try using pre-exhaustion or pre-activation while maintaining the ideal mechanics and technique for the dip.
To hit the triceps more effectively on dips and minimize stimulation to the chest and shoulders, perform an isolation movement such as triceps press-downs or triceps extensions (skull crushers) right before doing a set of dips. This will cause the triceps to fatigue and fail before the chest and shoulders give out.
More importantly, you'll have emphasized the tri's while keeping your movement pattern intact rather than wrecking your natural body mechanics with a mutated variation of the exercise.