Article from Warrior Talk News on the importance of moving your support (or weak) hand around for different types of shots. Basically, the author suggests using a forward hold--support hand far out on the rifle stock, with the arm almost fully extended--for longer distance shooting; a middle hold, closer to the magazine, for medium and short distance; and that a magazine hold--i.e., gripping around the magazine--should be limited to submachine guns.
I generally don't use a forward hold when using a rifle, because I tend to use my weak hand merely to support, or hold up, the rifle. When I do use the forward grip, I find that I get the best use by actually using my off-hand to pull the rifle back into my shoulder. On a rifle with a short fore-stock, I sometimes will actually put my first couple fingers over the front of the fore-stock to help with pulling it back.
If you look at some target shooters, when shooting from a standing position, they will bring their off-hand in close so that the elbow actually rests against the torso. The rifle then is cupped by the hand, or a tripod using the thumb (the back leg) and the first two fingers (the front legs). You can use this position comfortably with an AR or Mini-14 by resting the magazine in the palm of the hand. However, the 30-round magazines are too long. In that case, you would grip around the magazine well. This is not a mobile position and probably should not be used in anything but target shooting or hunting.