Every time you open the pot to stir you lose heat. I stir the grains in well when I dough in and put a lid on. I never stir it later. With BIAB you have a very thin mash and you don't have to worry about all the grains getting wet.
If you open the pot and find your temperature low, you will need to stir continually if you turn the burner on to add heat. From my understanding it is not to make sure the grains are getting wet, but rather to equalize the temperature throughout.