I know grips are a big topic of discussion for tennis teaching pros. There are textbook grips assigned to most shots and accepted by the majority of tennis professionals.
When teaching a "new" player, I try to stick close to the recommended grip. However, the more time a player spends on the court, the more likely it is that their grips will slightly change and evolve into what seems to fit their swings. Sometimes this happens and it is a problem for the player, especially when the grips become extreme.
However, most players' grips will change just slightly and still provide a racquet face that helps them hit a specific shot. For instance, a continental grip tends to open the racquet face slightly when hitting a slice forehand and can also help when hitting a slice backhand. This grip also helps create spin on the serve as well as allowing your wrist to get involved adding more miles per hour.
A western forehand grip can promote heavy topspin and varying the degree slightly in either direction can help determine the amount of spin that ends up on the ball. The purpose of the grip is to get the racquet face in a good position to hit the type of shot that you want. As long as you use a grip that is not fighting the shot you are trying to produce, you will likely be able to hit the correct spin or the shot that you were looking for with the help of a good swing.
Remember that there is not one perfect grip for every shot and you should use what feels comfortable as long as your shots are coming off the racquet the way you intended.
Steve Annacone, USPTA Elite Pro, is the Director of Annacone Tennis, www.annaconetennis.com and MyHamptonsPro, www.myhamptonspro.com throughout the Hamptons, NY. In addition, Steve and Miguel Coelho have introduced the JET (Junior Elite Tennis) program at the Tucson Jewish Community Center (Tucson, AZ) for high level players ages 8-18. Please contact Steve at firstname.lastname@example.org.