Forehands can be hit with grips ranging from Continental to beyond full Western. This range spans over 135 degrees of change in the angle formed between the plane of the palm and the plane of the string bed. (See Photo Tour of the Forehand Grips.)
In the last thirty years, the average position of pro forehand grips has evolved from between Eastern and Continental to fully Semi-Western. These grip changes have been an integral part of the trend toward increasing power and topspin. Generally, as the grip moves from Continental toward Western, the player finds it easier to generate topspin and handle high balls, but harder to generate slice and handle low balls.
Players like Stan Smith, who used a grip halfway between Eastern and Continental typical of his late 60s contemporaries, hit with considerably less topspin than, for example, Gustavo Kuerten, who hits with a Western grip. Smith also met the ball usually at or below waist height, while Kuerten meets the average ball closer to chest height.
In Smith's era, grass tournaments were much more prevalent than today, and a more Continental style was suited both to handling the low bounces on grass and generating slice to accentuate that low bounce. Many of the players who have found their greatest success on the Wimbledon grass, such as Martina Navratilova and Jana Novotna, have used forehand grips similar to Smith's, but a little closer to Eastern.
Now, with more tournaments on courts that produce higher bounces (clay and hard), Semi-Western and Western grips prevail. It's no surprise that Gustavo Kuerten has done much better on the French Open clay than on the Wimbledon grass.
Another factor in the evolution toward more Western grips is racquet technology. Hitting the heavy topspin for which the Semi- and full Western grips are designed requires a quick brushing action that is much easier with the light weight and large string beds of modern racquets.
Will grips continue to evolve more toward the Western?
The average of all forehand grips on the pro tours should move somewhat West as the older, more Eastern players like Novotna retire and are replaced by youngsters who grew up emulating more Western styles, but the remaining shift will be brief and slight. Racquets have gotten as light as they can while still having enough mass to hit effectively, and the courts of the future are unlikely to bounce any higher, on average, than those of today. Furthermore, most players find it pretty tough to get a forehand over the net with a grip more Western than the current Semi-Western average.
For some interesting details on this topic, check out this study relating grips and topspin.