For forehand with any grip, hold the racket quite relax so that the hitting arm is also relax and bent-- not tight, not straight!
For 1-handed backhand, hold the racquet quite tight so that the hitting arm is stiff and straight at contact (making an L with your arm and the racket).
For 2-handed BH: The double handed backhand is a combination of 1-handed BH (right hand) and lefty forehand: Thus, the right hand grip should be firm, the left hand forehand grip should be relax.
For serve: Hold the racket very loose as if it is going to fall off. A feeling of loss of control should occur. In that feeling of loss of control lies the control!
For volleys: Firm grip (and wrist) at contact!
Smash: Just like serve.
Will this trigger a new debate?