I stir before my first run off and let the grain bed settle for a few minutes.
Now I will poke into the hornet nest.
I stir my grain bed when I am sparging.
This prevents any channeling.
All this said you only need about 3" of grain bed for clear run off.
I recirculate while mashing and when I sparge I stir top of grain bed a few times.
I have a feeling (but not proof) that I get slightly better extraction.