The grip has an effect on clubface positions. Too weak of a grip, with your hands turned too far to the left, will lead to an open clubface through impact, and, hence, a slice. Although most pros are different, most of them have some form of a strong grip. They just differ on how strong, not necessarily if it's strong or weak.
So the basic idea with your grip is to have a strong grip. This means that your hands are turned more to the right, and underneath the club. This will make it so much easier for you to keep your clubface square throughout the entire shot. You will no longer have the tendency to roll the face open during the first three feet of your swing.
How strong your grip should be is in large part up to you. There are some great players on tour with super strong grips, like those with barely a strong grip at all. And a good way to measure a strong grip is to make the "V's" formed by the thumb and index finger point towards your right shoulder.
This is especially true with their left hand. With each of these players, you can see about three to four knuckles on their left hand. This gives them so much more leverage in terms of their wrist movement. They are able to wait to the last moment to release the club, which results in a surge of club head speed through impact.
So to sum up this entire grip issue, let me say this. We know that there are only two reasons why someone can slice the golf ball. It's either from going over the top, or having an open clubface at impact. Coming over the top is obviously caused from swinging above the plane on your downswing. Having an open face at impact is 95% of the time caused from a weak grip.
Therefore, if you look at the tour players, they do the exact opposite of the two causes of a slice. Just about all of them swing underneath the plane on the downswing, and just about all of them have some form of a strong grip.
Now, you know how important the strong grip is. If you are a good player and want to do golf swing well, remember to achieve a strong grip before changing all your clubs to new ones.