The key to good animation is timing. Slow in and out is a subset of this principle. The timing you use in an animation should give a sense of the object’s characteristics: how heavy or light is it, for example.
Timing can communicate a lot. A person blinking quickly might be seen as alert, whereas a person blinking slowly might appear tired. Changing nothing more than how quickly the eye blinks can help communicate the emotional state of the character.
More than anything, timing is something you should experiment with when creating transitions and animations. Trial and error and your own judgment will go a long way here.