I don't see where warming up the engine first would extend the life of the head gasket.
Having both M103 & 104 I can say this. The 104 had two head gaskets in about a 2 year span. The second gasket has been fine and the car currently has 106k on the odometer. The 103 has 152k miles and the head gasket has not been replaced since I've owned it. There is some seepage from the right front corner though.
92 Volvo 740 Wagon aka "Mutt mover"