Not a fact. A myth. It is imposible, even from rooted clone, to measure such things. Genetics off all plants are different. Even clones after rooting, have different genes than the mother. The only way to test such a thing would be to let one plant grow and trim one. But the plants would have to be absolutley identical in every way in order to prove or deisprove it's benefits.
What is certain is that lower buds, without light grow very small. Als,o cutting shade leaves will increase air flow and light penetration. If the lower buds have more light. They'll grow bigger. Thus ADDING to your yeild. The questions is would that increase make up for or even improve on the negitive effects of trimming.
There is no question, high stress or low stress methods of training required the plant be stressed somewhat. No question the less stress the better. But to say without a doubt that trimming is solely negative to marijuana is ludachris. But there are thosands of different genetics. One strain might halt growth when trimmed, others may take off. Cutting shade leaves that cover buds is going to increase the size of said bud. That's a fact. Whether it's beneficial or detrimental to the plants growth, is anyones guess. Just that, a guess.
My adivce would be to focus on your plants. Only the grower can say how HIS plants are doing. If you think it helps; Your their Dr. Greenthumb, you make the decision as to what's best for them. I would suggest getting light to all the buds. I always try to move or keep the leaf in someway that doesn't require trimming. But sometimes it's got to go. I would say if it's got to be cut, do it after watering.
Hope I've helped,
-Dude
