For the typical WW FH, secondary muscles associated with the biceps region are involved with the transverse or horizontal movement (both transverse adduction and flexion) of the swing. More or less, these muscles are important in creating the line of extension (or pull line) toward and through the ball.
The problem is that those muscles are usually not the primary movers of elbow flexion, therefore bicep curls and the such do not really train them significantly. But, then again, these are also "technique-oriented" muscles, and most people when learning the WW FH are looking to improve internal rotation (i.e. take bigger cuts) in their swing, rather than improving this aspect.
Directory of Tennis Warehouse Clubs (courtesy of Mountain Ghost)