I trim most lower branches, otherwise I usually have a hard time supporting all the branches and they end up breaking or not growing much at all. It really does depend on the species, the variety, and the growing technique. Plants grown close together will not get as much light penetrating into the canopy so side shoots won't grow as well as if the plants were spread out getting more side lighting. When growing under artificial lights, the top of the plant will get so much more light than the bottoms thats I never see much advantage to keeping the lower side branches. They just get in the way and encourage pest and disease near the soil. I've also had great success with adding side lighting to make a bushier plant, but once again these really need good support.