No, there are no absolute's in tennis, especially regarding grip for whatever stroke.
The vast majority of good tennis players hold with some form of continental grip, usually their volley grip, but not always.
For a flatter ball, some players switch slightly towards eForehand.
For a spinnier ball, some players switch slightly towards eBackhand.
And at least half the servers don't switch at all between different kinds of serves.
In your case, practice your serves, and use the grip that works for each serve.
Your opponent/s know what serve you are going to hit.