Remember that stirring the top third of the grain bed is simply a recommendation by Anvil and doesn't have to be a rule. I think though that the appropriate answer depends on several things: 1) composition of grain bill (lot of sticky grains in there like oats?), 2) magnitude of grain bill (16-19lbs?), and 3) recirculation rate.
Since the anvil is tall and narrow, if recirculating at too high a rate, the grain will compact and you will get a stuck mash. IMHO, you don't need to recirc at a high rate just to get good temperature stabilization. Go slow. If you have a LOT of grains and/or its sticky, I have tended to stir the whole grain bed with the exception of the last 20 or so minutes (I use a 90 minute mash schedule). If you sparge, having a nice and well settled grain bed helps for sparging. Also, if you do not have a bag in the malt-pipe I imagine that you would get more grain particles into the boil, but I use a bag so this doesn't concern me as much. So stirring the top third I think is a nice compromise to ensure the top of the grain bed is still loose and doesn't create channeling (again - recirc slowly). Given your grain composition and/or grain bill size, you may want to stir more of the grain bed or less overall. Cheers!