i just read an article in the Jan/Feb 2010 edition of Urban Garden that's called The 'Sweet Zone' - Pruning Indoor Plants for Larger Yields.
the "Sweet Zone" is defined as not too near and not to far space under the grow lamp that receives the most light energy without being so close to the lamp that the heat from the lamp interferes with the plant's health and metabolism. The aim of the indoor gardening game is to shape and position your plants so that many growth tips / fruiting sites are basking in the 'sweet zone'
Whenever a plant grows (roots, stems, leaves, flowers, fruits) it takes energy. The rate of a plant's development is limited by the amount of light (energy) it receives. This energy is distributed throughout the plant in order to grow and bloom. The aim of pruning is to focus this energy to where it's most needed - the fruiting sites in the 'sweet zone'
then it talks about the 'clean-up'. if left "un-cleaned", a light loving plant grown indoors will generally produce many small, low quality fruits and flowers, especially on the lower branches. The best time for the clean up is immediately after the initial stretch in flowering, but before the plant has shown much sign of fruiting. If too much is pruned off too soon, the plant will stretch even more and become somewhat leggy. If you clean up too late, you will be removing green matter that the plant has already invested a lot of energy into, some of which will be small fruits or flowers. The later you prune into the flowering stage, the more the plant is focusing its energy on generative growth instead of fruits. A good rule of thumb is to "clean up" the plant in the second week of flowering just as the first smalls signs of fruit appear and after the plant has stretched a little.
There is a fine line between pruning off too much and ending up with a very sparse canopy and a loss in yield, and leaving too much on an lowering the size and quality of the fruits.
Generally, I'm trying to prune off all branches lower than halfway up the plant. As the plants start to flower you should be able to crouch down and peer right through your garden underneath the dense canopy, but i've heard of people keeping only the top 3 nodes and cleaning up the rest.
not only will this focus the light energy on the tops making bigger, better quality fruits, but it also allows for better circulation throughout the garden.
while this article was not written about marijuana specifically, it's logic is sound to me.
http://www.marijuanapassion.com/forum/showthread.php?t=50787&page=3
here is my latest batch of clones that have been in flower for about 10 days and you can see how i'm doing it. my last batch of clones that i flowered got over 5 ft tall after the stretch. i have done some LST training on these guys, used bushmaster to reduce the space between the nodes (most of the tops it's hard to see the stem between the nodes, that's how close they got, i'll try to take a picture tomorrow to show how close they are) and put them into flower after they were only about 12 inches tall or so.