Which wrist action are you referring to? Roanic, Roddick and some others flex the wrist on the preparation (prior to the trophy phase). However, cocking the wrist on the serve usually refers to a wrist action after the trophy phase -- i.e., the radial deviation and wrist extension seen during the racket head drop and the early part of the upward swing to the ball. The wrist uncocks on the way up to the ball (to a fairly neutral position at contact).
Every tool is a weapon -- if you hold it right. (~Ani DiFranco)