I top my plants as well. I have a 12 foot trellis and prune them once they reach the top. As stated above, the lateral shoots will be stimulated by this and go crazy. I've done it both ways, and I don't think pruning will increase your harvest, but it won't really decrease it either. The only reason I do it is to make the plant more bushy so that more moisture gets trapped around the leaves. I have an easier time fighting off the spider mites this way. They can be a real nuisance for me come July and August.