I just let the players decide. My take on this is that this is flavor for the players, so I give them complete control. They can choose any of the manifestations listed, they can roll once, or every time they cast it, or they can make up their own manifestation. But they cannot cast a spell unless they describe how it manifests
I am similar. Sometimes, in the heat of the moment, we forget manifestation. Otherwise, one of two things happens:
(1) The first manifestation rolled is so damn appropriate for the character that we just keep it as that character's signature, or
(2) We reroll every time because the players seem to enjoy it. They roll.