To me that's the logical choice.
Each grade get 11 fresh players and your subs for reserve and first back up from thirds and reserves. Yeah sometimes each grade can't get 11 fresh but you certainly don't risk injury to your first grade striker to play thirds whilst you have 14 or 15 blokes in third grade.
I would have hated to be the bloke who regularly plays striker in third grade. Sitting on the bench whilst the first grade striker is out there and you not being able to help your mates qualify for the finals. Those on the bench getting half a game in thirds whilst a couple of others are playing 2 games, I would be filthy if it was me.