On return of serve:
Some will wait in a BH grip, and some will wait in a FH grip.
But it is better to understand the serving patterns of the server. If he serves more to your BH, then wait in a BH grip; if he serves more to the FH then wait in a FH grip. If the server is unpredictable, then of course, try to read the toss, and angle of racket prior to contact, and go to the correct grip.
On ground strokes rally: It is better to change grip during rally. If you have hit a very good deep cross court, you may stay in a FH grip, because in this situation it will be difficult for your opponent to hit down the line to your BH.
If your shot is easier/shorter to his FH, you may then switch to BH grip because he is likely to hit to your BH.
Similarly, if you have hit a great cross court to his BH, wait in a BH grip because in this case he will find it difficult to go down the line to your forehand.
Your own shots will allow you to change your grip to the anticipated return from your opponent.
Again, if you watch your shot, the body position of your opponent, and the racket face just prior to during contact, you will have enough time to change to the correct grip.
With experience, the grip change becomes automatic.
Good day,
Mahboob Khan