Ok so with all shots you should be quite relaxed in your grip, definitely on high spin serves this is important to avoid silly mistakes. However squeezing a little bit with your first finger and thumb as you contact the ball is also helpful, as long as you don’t press to hard and get tight. This squeeze as you contact helps to get the momentum and contact you want through the shot, but your other fingers should still stay pretty relaxed.
Even on topspin and block shots this squeeze as you contact can help, its just important not to tense up and keep the bottom 3 fingers loose around the bat.
