One of the common complaints about aquarium plants is that, as they grow taller, they lose their lower leaves. This is especially true in lower light situations. Most people will recommend trimming the plants, removing the bases and replanting the tops. In a mature, heavily-planted aquarium that's probably the right idea. But when you are trying to grow a stock of plants, it isn't the best idea - in that case, the objective is to maximise your stock of planting material.
When you prune a plant, it will normally produce new shoots from an existing axillary bud. These buds are located at the point where the petiole of a leave joins the stem (or where one used to be before the leaf was shed). When a stem it pruned, one or more axillary bids start to grow. This can produce a bushier plant, but the appearance isn't always what you would want, since you end up with a distinctly smaller shoot coming off of a larger stem.
If the branch develops from the topmost axillary bud, there's a chance that it will develop into a new leading stem. But will that really create the desired look? This is far more important for foreground plants than it is for background plants, of course. From this perspective, it seems like it would make sense to cut plants back are low as possible, leaving an almost indistinguishable old stem. The obvious problem there is the danger of cutting it back too far. The plant can only produce new stem material if it has the resources to do so. How little is enough? That's not only going to vary from species to species, it's also going to vary from individual to individual. A small scrap of Java fern leaf will grow into a new plant, but how little Ludwigia stem (to use the example in these pictures) will produce a new plant?
There is, of course, another problem as well. Large, healthy plants do not appear out of nowhere. If you prune too heavily you may end up with a large stock of skinny, sad-looking plants.