In "modern" framing techniques there is a bit of worry about shear bracing. This is typically satisfied by the use of plywood panels and a specified nailing pattern. Not so on a timber frame roof that is sheathed with boards (butted, shiplap, t&g). Nearly every photo I see of the underside of a timber frame (boarded) roof shows no evidence of diagonal bracing. So...

What is the current wisdom on the need for diagonal bracing of a timber frame roof with board sheathing?