Unless you are an anatomical fluke of nature, your longest finger is your middle finger. Thus, with few if any exceptions, when a pitcher throws a baseball, the last finger in contact with the ball is the middle finger. No matter how a pitcher grips the ball, the middle finger is the final contact point. So, the emphasis on grips is overrated, in this sense: because the final contact point remains the same regardless of grip, it is as important to vary the angle of the hand at that release point as it is to change the grip.

For instance, a 2-seam fastball is released with an over-the-top hand angle. In other words, the thumb is under the ball. A sinker, by contrast, is released with a pronated hand angle, i.e., the thumb remains under the ball although at an angle more to the outside of, or away from, the body. A so-called knuckle-curve is released with a supinated grip, meaning the thumb is on the inside and upper portion of the ball, almost as if the pitcher is throwing a football.

Note: It is paramount that a young pitcher, in particular, recognize that each of the pitches described above, in addition to many others, may be released with the same motion as a typical 4-seam fastball. No twist or curvature or awkward movement of the hand, wrist, arm or elbow is required in order to impart movement of the ball. 

Focus as much on your hand angle as your grip, and see the improvement. The simpler a pitcher makes his task, the more he can focus on excellence. Throw strikes, throw great, and be great.