The question is open ended depending on age and the pitcher himself/herself.
For example, a pitcher in the youth game - 12 and under, I see tosses a fastball with the best grip that maintains control in the hand. On the other hand, the size of the hand and other physical endowments would preempt any grip, regardless of the age.
For those pitchers with the gift of mental reasoning, a grip with finger pressure here-n-there, and a keen sense of observation, all coupled with a .. "Whoa Nelly.... did I just see that.." when the ball is sent down range .. reinforces an examination of .. " now, how did I just hold this thing?" I use to sit in a lawn chair and watch guys in practice toss the ball around for literally hours ... look at the ball, spin it in their hand, toss the ball some more, re-grip the ball just a little different, and so on. I could always tell when something new was added to the complexion of whatever pitch they were messing with. Their face would light up ... and bingo.... "give me the ball back... quick!"
Pitchers who literally survive out there, don't remain static with just one grip, regardless of the pitch and how the ball is held. These guys reinvent themselves on a regular basis - always experimenting, always testing the waters.
This is an outstanding question and one that deserves a "good thinking."