Based on my experience and what I've read, they tend to produce the most cones near the tops of the plants, so I wouldn't advise pruning them or you're most likely going to substantially reduce your yield. I have mine growing on a trellis that's about 8 feet tall. When they reach the top, they cascade off the other side. So you don't have to grow them 20ft tall. I've also heard of a lot of people training them to grow laterally along a fence.