Ok I think I get what you're talking about.
What you're aiming for, no pun intended, is to have your cue go straight through the CB. So your not looking for fluidity in the wrist up and back but rather from side to side, so that the cue moves freely and doesn't dip or raise as you stroke? right? Yes that is a good thing, the straighter you can keep your cue throughout the stroke the more consistent you will be in your CB control.
However, if you're letting the cue do the work and not forcing it through the CB that will happen naturally. If you are trying to force your wrist to be loose then it won't. When you grip the cue hold it loosely to the point you ALMOST feel it slip in your grip as your cue makes contact with the CB.
If you do that it should allow the side to side motion of your wrist to remain loose. When I say side to side I mean relative to your arm, not relative to the line of the cue.