I wouldn't prune unless absolutly neccesary. Are you pruning so your plants won't get that tall? Pruning only hurts your plants by stressing it making it work harder to grow. It dosnt increase yield becuz pruned plants grow slower than not pruned plants. If you want 2 increase yield are save vertical space, tie your plants sideways with shoe strings so that the internode arms turn into little plants. INCREASE YIELD SUBSTANTUALLY