It is important to set the centre main bearing that holds 2 halves of the 2 thrust washers on each side of the bearing. The bearing bolts should be lightly tightened then the crankshaft moved the side to side which will line up both halves of the washers then the bearing bolts final tightened to the specified torque. This should be done whether the washers are renewed or not.
I have also read that many engine rebuilders will swop the washers around if only lightly worn as most of the wear will be on the rear facing washers due to the force applied when the clutch is used to change gear.
MGB tips mods and maintenance 