This method of setting the command speed is ONLY applicable to Flap Extension using the Alternate System.
The 737 FCTM states: "When extending the flaps using the alternate system, the recommended method for setting command speed differs from the method used during normal flap extension. Since the flaps extend more slowly when using the alternate system, it is recommended that the crew delay setting the new command speed until the flaps reach the selected position. This method may prevent the crew from inadvertently getting into a low airspeed condition if attention to airspeed is diverted while accomplishing other duties."
There is nothing here that talks about flap asymmetry, or other such nonsense.