you never grip, with any part of your leg, lower or upper.
Having your stirrup longer does not mean that your leg will be longer, and that you'll post better. On the contrary, it often makes a person end up with the heel coming up , the toe going down and the person losing the entire lower leg and posting off of the knee. It's a very weak position.
There is an excellent article in last months Ecclectic Horseman magazine, by Wendy Murdoch, about the importance of "heel down" and what is really meant by that. It is all about getting the body weight to flow down and out the heel, not to push against the stirrup. The angle should be about 92 degrees as measured behing the knee (the angle of the upper to the lower leg). That right angle can be incorrectly placed too far forward, or correctly placed under the riders body, but, making the stirrups so long that the angle is much more open does not make a correct position.