Most of the time your grip pressure should be very relaxed (fairly loose) so that your forearm muscles are also relaxed. Grip pressure will naturally increase as the racket head is accelerated forward (or upward foe the serve). Grip pressure studies done on pro players indicate that they employ about 1/2 grip strength whenever the racket head is accelerated. Once the the follow-thru is (nearly) complete, the grip is, once again, relaxed.
For volleys, players are often taught to squeeze the racket grip to accelerate the racket head in order to meet or "punch" the ball.