I don't have a HERMS so I might be off, but when I fly sparge I let the mash settle through the 1 hour mash rest and it remains settled while I sparge. The settling of the grains create a filter of sorts so you don't get grains into the boil kettle (sorry if I'm telling you what you already know). You might get a bit more efficiency by stirring, but probably not necessary, and probably a bit more risk of a stuck sparge... plus less work is always a good thing in my book.